Why Do You Need a New PBX?

First of all, be very clear why you are going to buy a new system. Valid reasons may include:

  • System Out-of Date and Difficult to Arrange for Maintenance

Your PBX is so old that it can no longer be maintained properly. This may mean that all the technicians capable of working on the system have retired. Or it may mean that the manufacturer is not longer making replacement parts for the system. Be sure that both of these "facts" have been carefully researched, as they are seldom the case. Most telephone systems, though aging, can be supported indefinitely buy finding the right resources. One company now even advertises that it specializes in the support of "vintage telephone systems."

  • Few of these PBXs installed. Maintenance support unavailable

One can make a good case for not being the first one on your block to purchase a particular new telecommunications system, either from a familiar or a new manufacturer. If few organizations buy the system and it is not a commercial success, you may be stuck with an unsupportable system that is not very old.

For example, Avaya (formerly called Lucent. Lucent was formerly called AT&T) used the name Merlin for several different systems. Avaya also came out with the System 25 and then discontinued it several years later, so although there are still some System 25s around, the companies who maintain them are few and the cost of maintaining them can be high.

  • System at its Maximum Capacity

As PBXs reach their maximum capacities, particularly if they have not been efficiently maintained, they tend to not operate as well as they had in the past. Problems arise, such as callers being lost when transferred. Also every system does have a maximum capacity beyond which it cannot expand.

You may need the capacity because you are adding people or you may need it to because to wish to add capability to your system such as voice mail or networking with other locations.

  • System lacking in desired capabilities

Before you buy an entire new system for a certain capability, make sure that there is no other alternative and that you really need this capability. Some older PBXs cannot display the telephone number of the person who is calling you from the outside (Caller ID) on your telephone. This is one function that organizations may replace the system to obtain. Often a system upgrade, while it may include the purchase of hardware and software, will be less costly than purchasing an entire new system.

  • System problems that cannot be resolved

Replacing a system to solve problems without knowing what is causing them never makes sense. The same problems may be there after the system is replaced since they may have been a result of (1) poor system design, which you may have inadvertently replicated (2) outside line problems (3) cabling problems or (4) power problems, to name a few.

Before replacing a system, it is important that the problems have been identified and it is agreed that they cannot be fixed. This often takes patience, money and skill on the part of whoever is diagnosing the problem. At worst, it may take a technician, an administrator, an engineer and a consultant. At the end of the process, the outcome may point to one of the other reasons for replacement such as the system being too old to maintain or at maximum capacity. Or if the problem is not with the system, then a considerable expense may have been avoided.

  • Poor experience with the maintenance company

It is always advisable to purchase a PBX that can be supported by a number of different companies in your area. This way, if service is not satisfactory, you have options. Some organizations are forced to replace the PBX if no such option exists.

Top 10 (+1) Places To Go Wrong When Planning For And Purchasing A New PBX

Here are the areas where organizations most often go wrong in identifying their requirements:

  1. Underestimating time & effort

Underestimating the time and effort needed to do this. What you might save at the front end will be more than offset by the cost of trying to fix things after the fact, if indeed they can be fixed at all.

  1. Ignoring the details of how the current system is configured

Failing to develop a complete understanding of how your current system is configured and how circuits and peripheral systems are connected to it. This often results in not buying sufficient capability and capacity with a new system, then encountering costly additions after the initial system purchase. Do not assume that this information is readily available. Someone who knows the questions to ask and can document the answers in a clear and detailed manner needs to lock himself in the telecom equipment room for a day or two with the PBX technician.

  1. Not focusing on how your organization is using the current system

While it takes a lot of time to investigate how your organization is using the functions of the current system, this is time well spent. For example, "How do you transfer calls from one telephone to another?" If you only press the transfer button once now to transfer a call and a new system requires that you press it twice, this may be viewed as a step backwards (which it is!). The intercom is another capability that is often overlooked and can lead to disappointment when a new system is installed. Many manufacturers have focused their energy on accommodating "new technology" at the expense of making the system easier to use.

  1. Assuming uniform capabilities among manufacturers

Assuming that all manufacturers telecommunications systems pretty much have the same capabilities. They don't.

  1. Not completely thinking through Call Coverage

Not having enough discussion and documenting how your organization will cover calls under a variety of circumstances. (i.e. when called person not at the desk, when called person is on another call, day, night, weekend, etc.). You may find that you have purchased a system that cannot accommodate your plans.

  1. Planning Call Coverage with Too Much Detail before the final system selection.

Since each different system (even those from the same manufacturer) has its own proprietary call coverage capabilities and uses its own terminology, it is a wasted effort to plan call coverage down to the details of every telephone at every desktop. Trying to change it after the fact can result in a cumbersome call coverage set up that results in poor service to callers and staff.

  1. Not communicating your requirements well to the suppliers

Collecting a lot of good information, but failing to communicate it well to the telecommunications system suppliers bidding on your project. It is suggested that this communication be both in writing and in a lengthy discussion to confirm what is in writing, to avoid misunderstandings when it is too late to do anything about them.

  1. Falling in Love with Technology

Failing to balance a desire for experimentation with new technology with the need for traditional reliability.

  1. Ignoring the Telephone at the desktop

Paying too little attention to the telephone instrument that will sit on everyone's desk while focusing on technology and backroom equipment. Most people judge the system by how well they like the telephone.

  1. Not Budgeting Enough

Not budgeting a sufficient amount of money for the purchase, including a variety of professional services skills needed to implement it such as consultants, system designers, programmers, project managers and trainers.

  1. Spending too much

Not understanding how much the system will cost, what is needed and what is negotiable. Therefore many organizations spend up to 100% more than is required.

Expense Control Function of the PBXs

Every time a telephone call is made, money is being spent. Many of the PBX functions help an organization to control this expense.

Toll Restriction

Toll restriction enables you to program the telephone system so that each extension has what is called a class of service. Each class of service designation enables the extension to call certain areas and restricts it from calling others. The most sophisticated systems can be programmed to restrict the dialing of specific telephone numbers. Others can restrict by area code or by area code and exchange. Figure 1 shows a typical scenario.



Internal Calls Only



Local Calls Only to 212, 646 and 917



Local Calls to 212, 646, 917, 718, and 914



Local and Long Distance Calls Within NY Metropolitan Area 212, 646, 917, 718, 914, 203, 860, 516, 972, 908 and 201



Local and Long Distance Calls to Continental US



Local and Long Distance Calls to US and Canada Only



Unrestricted - Can Call Anywhere in US or International

Figure 1: Classes of Service for Toll Restriction (Example)

You may also use class of service to give each extension access to certain system functions and restrict it from using others.

You may hear the term six digit toll restriction, meaning that the first six digits dialed (the area code and exchange) can be restricted through programming. Ten digit toll restriction enables you to restrict dialing to a specific telephone number.

Automatic Route Selection

Most PBXs and some key systems have the capability for ARS (Automatic Route Selection)or LCR (Least Cost Routing), which are essentially the same thing. Many larger telephone systems have separate outside lines that, when used, will result in a lower cost for certain telephone calls. For example, your company may have a direct line (dedicated line) connecting you to a long distance carrier. When a call has been dialed from your telephone system, the ARS recognizes where the call is going and sends it over the lowest cost route, based upon how the system has been programmed.

In the 1960's, '70's and '80's many companies had WATS lines (Wide Area Telephone Service). WATS lines were separate outside lines enabling calls to be placed at a discounted rate to specific geographic areas within the U.S. (Band 0 through Band 5, as the areas were called.) It was during this period that the ARS was most important. Now the decisions are simpler.

Telephone systems without ARS can get around this by assigning access codes to separate groups of outside lines called trunk groups. In this scenario, you may dial 9 for local calls and dial 8 for long distance calls to access a separate group of lines.

Call Accounting

Most telephone systems also have the capability to provide information on the calls being made through the system. This includes the time the call was placed, the duration of the call and the telephone number dialed. This is called SMDR (Station Message Detail Recording) output, Call Accounting output, CDR (Call Detail Recording) output, or AIOD (Automatic Identification of Outward Dialing), an out-of-date term seldom used. It's important to understand that the PBX provides the raw data only. In order to do anything with the data, it must be captured and processed into a usable format.

In many organizations, the SMDR output is stored in a buffer device that then is polled by a call accounting system, usually on site. Call Accounting is usually PC-based. It accepts the call information from the PBX. It then assigns a cost to each call that approximates the true cost of the call and sorts the calls by extension number. The costs for each extension may also be grouped into department reports, often provided to each department manager.

Calls may also be sorted by account codes, numbers associated with certain clients or projects of the company. Thus, a law firm will know how much is being spent on each client and may use the information for billing call charges back to the client. Under these circumstances, the PBX requires the dialing of an account code before each outgoing call is made. The PBX keeps the account code associated with the call and sends this information to the buffer device.

Traffic Study

Most PBXs provide information on how the outside lines in the system are being used (call volume on each outside line). This is called a traffic study. Although the call accounting system is capable of providing a traffic study, it is not always set up to do so. Instead, the telephone installation and maintenance company is customarily called upon to poll the PBX for a period of time, often a week, to provide the traffic information. When requesting a traffic study, it is important to be very specific about the information you want and what your objective is.

You may have had a new telephone system installed with 20 outside lines for which you are paying $30 each per month (20 x $30. = $600). Now it's a year later and you want to find out if you really need all of those lines.

A traffic study runs for one week and tracks all incoming and outgoing calls. The results indicate that at the busiest times of the day, no more than 10 of the lines are ever in use. Being conservative, you decide to leave 15 lines in place and remove the other 5 (5 x $30 = $150), thus saving your company $150 per month.


When you remove outside lines, remind your telephone installation and maintenance company to reduce your maintenance cost, since it is often based upon the number of ports used for outside lines. You must also have your telephone system re-programmed so that it will not be looking for the missing lines when someone dials 9 or, in the case of your DID trunks, when a call comes in.

Here's another situation for which you may wish to run a traffic study. Callers to your organization complain that they are reaching a busy signal. The one-week traffic study shows that all 20 outside lines are in use ten percent of the time, confirming that during this time callers cannot reach your company. You order 5 more local outside lines from the local telephone company and have them connected to your system by the telephone installation company. You may need to buy an additional circuit board for the system to accommodate these new lines. Several weeks later you run another traffic study that shows that at the busiest times of day, no more than 22 lines are in use. Callers cease to complain about busy signals.

The traffic study also measures call volume on T-1 circuits.

It's a good idea to have a traffic study run on your telephone system at least once a year. You may negotiate an annual traffic study as a part of your PBX maintenance contract. Or you may learn how to run the traffic study in-house with your call accounting system.

MAC (Moves and Changes)

Many companies spend thousands of dollars moving telephones around the office and changing the extensions or features on the telephone. This activity is MAC work (MAC = moves and changes). It can be provided by your telephone installation company. Most PBXs enable you to do some of your own MAC work using a PC with software that makes program changes to the PBX. In some older systems, the changes may be done with a keyboard only or a dumb terminal.

For example, two executives are exchanging offices. It is necessary to change the extensions appearing on the telephones in each of the offices, on the secretaries' telephones, and on the telephones of the groups backing up each of the secretaries. If the telephones are all in place, these changes may all be done from the on-site MAC terminal, without calling in the telephone installation company. It is always advisable to inspect the individual telephones to be sure that the changes took effect and to change the paper labels which most telephones still use. (Note: Some newer systems enable users to take the telephone with them and plug it in in the new office, retaining all of the extensions and features.)

Another example of a PBX change which can be made on-site is that of changing the toll restriction on a telephone. Your call accounting system may indicate that a lot of long distance calls are being made from the lunchroom telephone. Since employees are permitted to make local calls only, that telephone is now reprogrammed so that only calls to the local exchange can be made. All other call attempts will be routed to the switchboard attendant.

An advantage of having a MAC terminal on site (and someone in your company trained to use it) is that it will provide you with current information about how your system is programmed. Otherwise, you may be dependent on your telephone installation company or may even need to collect the information manually. MAC terminals may enable you to obtain the following:

  • A list of all extensions in use in your telephone system.

  • A list of all spare (unused) extensions in your telephone system.

  • A list of all outside lines (trunks) in your telephone system. This will be by the trunk number assigned to each outside line in the PBX. It will not provide the actual telephone number assigned to the line by the local telephone company, although there may be a field for this to be manually entered.

  • A company telephone directory listing name, department and extension number.

  • A list of all extensions with their associated class of service, meaning which areas can be called and which system functions can be accessed.

  • A list of all classes of service in use in the system and what they mean.

  • A list of how each extension is programmed to forward calls under a variety of conditions, including when the extension is busy and when it is not answered.

  • Information on how each of the slots and ports is used in the PBX control cabinet.

  • Identification of spare slots and ports available for expansion.

System Features Accessible To The Desktop Telephone Working With A PBX

Outgoing Calls

Placing a telephone call to an outside location: On most business telephone systems (PBXs) you dial 9, wait for an outside line dial tone, and dial the telephone number. You then hear a ringing signal that is sent to you from the local telephone company central office to let you know that the other end has not yet answered. You are not hearing the actual ringing of the telephone you have called. On other systems (smaller Key Systems), when you pick up the handset you already have an outside line dial tone and need only dial the telephone number, without dialing 9. Dialing 9 has become an accepted convention. There is not a technical reason that a telephone system cannot be set up to dial a different digit for dialing out, provided it has not been used for any other function.

Intra-office Calls and Intercoms

Placing a call to another telephone inside the office: Most business telephone systems enable you to reach any other telephone on the system by dialing the three- or four-digit extension number of that telephone. The called telephone may ring differently (probably a different cadence) to indicate an intra-office call. The display of the ringing telephone may indicate the name of the person who is calling.
There are many different types of business intra-office communications. In addition to simply dialing another extension number, most systems have other capabilities sometimes known as intercoms. Some are separate group intercoms for a specific department, or there may be two-way "boss-secretary" intercoms. The "boss-secretary" intercoms are designed to emulate the older style button and buzzer type intercoms that worked very well. These intercoms took up two buttons on a multi-button telephone, one for the buzzer and the other for the intercom path on which the conversation between the boss and the secretary took place.
Some intercoms have voice announce which enables the voice of the person who calls you on the intercom to speak to you and you to respond back, without having to lift the handset. Off-hook voice announce (less common) enables someone within your company to speak to you through the speaker of your telephone while you are on an outside call using the handset.
Dial intercoms are subgroups within a business telephone system, enabling members to call each other by dialing one or two digits without having to dial the complete extension number. The call may also ring on a separate button on the telephone, rather than on the button of the main extension number.
Some business telephone system intercoms have paging, enabling an announcement to be made from the speaker of every telephone at the same time (no more than 15 telephones simultaneously on most systems.). Other systems enable access to a separate overhead paging system.
Smaller business telephone systems (Key Systems) tend to have more flexibility in terms of the internal communications options than do the larger systems (PBXs).

Incoming Calls

Receiving a telephone call from either outside or inside the office: If someone is calling you, you answer by picking up the handset and saying "Hello." Some systems enable you to answer by just pressing a speaker button and saying "Hello" without lifting the handset. Other systems enable you to just say "Hello" without touching anything (seldom used but has application in environments where the recipient can't lift the receiver himself). Incoming telephone calls may provide the telephone number or name of the caller with Caller ID providing your system in equipped to accept and display the number (or name) of the person calling.

On Hold

Putting a call on hold: Many telephones are equipped with a button, often red or orange in color, which enables you to put a call in progress on hold. This means that the call is still at your telephone. The caller cannot hear you, so you are free to do other things such as call someone else, take another call, search for a file or gather your wits. Multi-line telephones almost always have a hold button. Some single-line telephones have one as well.
Some telephones have hold recall, which signals you with a ringing or other sound when you have left someone on hold too long.
Other telephones have individual hold, or I-hold. This means that if you put a call on hold at your telephone, no one else in the office that has the same line or extension can take the call off hold from any other telephone.

When a call is on hold, the light on the telephone where the call is holding rhythmically flickers to distinguish it from a call in progress (usually a steady light) or a new ringing call (light flashing on and off). Most systems also provide the ability to distinguish a call you put on hold from a call someone at another telephone put on hold it you both have appearances of the same extensions or outside lines on your telephones.

Call Transfer

If you wish to send a call to another telephone within your office and the extension the call is on does not appear on that other telephone, you must transfer the call. Most telephones are equipped with a transfer button that you press prior to dialing the extension number to which you want to transfer the call. You announce that you are going to transfer the call to the person at the other extension, and when you hang up, the call is transferred. If you do not announce the call and the extension to which you have sent the call does not answer, the caller may end up back at your desk, at the switchboard or in Voice Mail. This depends upon how the system is programmed.

Conference Calls

Most telephone systems are equipped with a conference button on the telephone. This enables you to set up a conference among three or more people, connecting people within your office to others outside the office. Systems vary in the number of inside and outside callers that can be conferenced. Typically, it becomes hard to hear on a conference call with more than three participants unless you are using specialized conferencing equipment that is separate from the telephone system.
For conferences of more than three or four people, it is advisable to use special conferencing equipment (called a conference bridge) or an outside conferencing service. It is important to know how to drop off one of the conferees from your conference call without ending the call. Not all systems can do this. If you have set up the conference call from your telephone and you hang up, you may disconnect the other call participants.

Last Number Redial

Many telephones store the telephone number you have just dialed so that if you reach a busy signal and wish to try again you need only press the last number redial button. On some telephones you do not need to lift the receiver to do this. Pressing the redial button will activate the speakerphone. Some telephones have a similar feature called save and repeat. This usually takes up two buttons and enables you to place other calls while the telephone still retains the number you want to retry at a later time.

Speed Dialing and Automatic Dialing

These capabilities enable you to store frequently called telephone numbers in your telephone. Then you need only press one or a few buttons rather than dialing the entire telephone number. If you have spare flexible feature buttons on your telephone, these can often be set up to automatically dial a telephone number by pressing just that one button (typically called Automatic Dialing or Autodial). Or there may be a button for speed dial that you press, followed by a one or two digit code (on the dial pad), which represents the stored number. On many business telephone systems there is station speed dial, specific to a particular telephone, and system speed dial, accessible to authorized telephones throughout the system. If the telephone system requires dialing 9 to dial out, you may have to program the 9 into your speed dialing, although some systems have built in intelligence to add it for you.

Call Forwarding

If you are not going to be at your desk, some telephones enable you to forward calls to another telephone either within your office or at an outside location. Many systems can forward your calls to different destinations depending upon whether your telephone is (1) unanswered or (2) busy, and whether the caller is (3) inside the office or (4) outside.
Some telephones are set up with a button that, when depressed, will send your calls directly to Voice Mail if you are not at your desk. This prevents the caller from having to wait for your telephone to ring several times before going to Voice Mail. Off-system call forwarding enables you to forward your office telephone to your cell phone or home telephone.

Call Pick Up

This function enables you to answer another ringing telephone in your office even though the extension number that is ringing does not appear on your telephone. This is usually accomplished by pressing a button on your telephone labeled call pick-up. If you have a display you may see the name of the person whose call you have picked up, which will enable you to answer appropriately, "Rose Bodin's office," rather than just saying "Hello." Group call pick-up lets you answer any ringing telephone in your pre-selected group. Directed call pick up requires you to know the extension number that is ringing and to dial it after pressing the call pick up button.


In business telephone systems, it is customary for more than one telephone to pick up the same extension number or outside line such as a secretary's telephone being able to answer the boss' extensions. Privacy prevents someone else who has the same line or extension from inadvertently cutting in on your conversation. If you want to let him in, you may do so if you have a privacy release button. Not all telephone systems are automatically equipped with the privacy feature.
Some systems have automatic privacy, while others require that a separate button on each telephone be activated to ensure privacy. A separate button called privacy release can let a person at another telephone in on a conversation if you want him to join you or listen in.


Most telephones working with a PBX have a release button. When you press it, the effect is the same as if you hung up the receiver and then lifted it up again. Pressing release disconnects a completed call and gives you back the dial tone to place another call.

Do Not Disturb

When the "Do Not Disturb" button is depressed, on most systems, you will not receive internal calls or intercom calls. This button often overlaps in function with a button that, when depressed, sends callers directly to your Voice Mail box without ringing your telephone.

Call Park

This enables you to ask a caller to wait, place the call in limbo in a numbered "parking place" in the system (the call will no longer appear on your telephone when parked). You can go to any other telephone on the system and retrieve the parked call by dialing the parking place number. This is also used by switchboard attendants who park calls and then announce over the office paging system, "Tom Taylor, dial 23." 23 is the parking code.

Call Waiting or Camp-On

If you are on a call and do not have a second extension, a second call may still get though to you using Call Waiting (also called Camp-On in older systems). You will hear a tone indicating a second incoming call and may press a button or the switch-hook to answer the second call without losing the call you're already on. The first caller cannot hear your conversation with the new caller.

All of the previous capabilities are features of the PBX accessible from the desktop telephone, usually accessed by pressing a specifically programmed button on the telephone, but sometimes requiring the dialing of a numeric access code as well.
There are also features that have to do with the physical makeup of the telephone itself such as the display and the speakerphone. 

PBX Sizing and Growth

When planning the purchase of a PBX, growth capability is a major consideration. Although they may be modular in design, in terms of telephones and outside lines that can be connected to the system, PBXs have a maximum capacity. In most systems, one port represents the ability to connect one telephone or one outside line. A 400 port system can accommodate a total combination of 400 outside lines and telephones.

There are no precise ratios in terms of how many outside lines are required for the number of telephones. A conservative ratio would be 25 outside lines (for incoming and outgoing calls) for every 100 telephones. If there are not a lot of calls, 10 lines may be sufficient. If it's very busy, more than 25 may be needed.

There are statistical tables and software programs available to enable you to judge the total number of outside lines needed. Using these statistics to determine the number of outside lines required is called traffic engineering. In order for this to make sense, you need to know how many calls of what duration will be handled during the busiest hour of the day. Most people do not have this information, and therefore use judgment combined with trial and error in estimating the number of outside lines needed.

In the world of telephone traffic engineering you may hear the term CCS which stands for 100 call seconds (C being the Roman numeral for 100) or, more simply put, 100 seconds worth of telephone calls. 36 CCS or 3600 call seconds are equal to one hour's worth of calls known as an Erlang. The statistical tables or software programs express call volume in this manner to determine the number of outside lines needed to handle this volume of calling. Something called grade of service is expressed in terms such as P.01 meaning that, statistically speaking, one percent of all calls will be blocked.

Another consideration in planning for the system size is that Voice Mail, paging systems and other peripheral systems also take up ports in the PBX.

Additional Considerations On PBX Sizing

Each telephone system from each different manufacturer has maximum capacities for both the number of outside lines and desktop telephones it can support. This is determined by the particular design of he manufacturer and has to do with economics as well as technology. Smaller capacity systems with fewer capabilities are sold at lower price points. As an organization grows, they may reach the point where their current system is "maxed out" and needs replacement, enabling the supplier to sell them a new system. The more basic design of these systems also enables them to be sold at a lower cost. In general, the larger systems (over 100 telephones) may be more flexible in terms of expansion, but even so, the cost of expansion can be significant and may involve the change out of both software and circuit boards. If you are considering a system from a particular manufacturer, looking at their entire product line can provide you with insight into their strategy for providing systems to organizations of varying sizes. A few of the larger manufacturers (such as Avaya, formerly Lucent, formerly AT&T and NorTel, formerly called Northern Telecom) provide systems for 5 telephones up to systems with 10,000 telephones or more. While the smaller systems are overall less profitable, there has been a need for the big companies to compete with the manufacturers of smaller systems to keep their customer base intact.

Most very large organizations have small locations as well, so for a manufacturer to provide complete coverage, it must offer the smaller systems.

Many other manufacturers target a more limited range. There are many more manufacturers of smaller telephone systems (either PBXs or more basic systems sometimes known as "Key Systems") than of large systems. Only 4-5 companies go up to the 10,000+ telephone sizes.

PBX Circuit Boards

Each shelf in the PBX is designed to accept a certain number of printed circuit boards. These are also known as circuit cards, or just boards, cards or circuit packs. Just as each PBX manufacturer puts its system together somewhat differently, the names of system components may also differ, so it is important to clarify what the terminology really means.

The devices used and the manner in which they interact differ from one PBX to another, but the basic building block of the PBX is the printed circuit board.

There are different types of circuit boards in the PBX:

Both-Way Trunk Circuit Boards

Both-way trunks (also called combination trunks) are outside lines that may be used to receive incoming calls or to place outgoing calls. There are ports (electronic places) on each circuit board, one port per trunk. Some systems have 32 trunks per circuit board. Others have sixteen, eight, four or two. A few older systems use two ports per trunk instead of one.

Every time you add outside lines, determine whether or not you have sufficient ports on the circuit board to handle them. If you do not, you need to buy another board. Trunk circuit boards can run from $1,000 to $3,000.

Sometimes both-way trunks are used for outgoing calls only and may be referred to as DOD (Direct Outward Dial) trunks.

DID Trunk Circuit Boards

DID (Direct Inward Dial) trunks are a special type of trunk used for incoming calls only. They enable the direct dialing of each individual desktop telephone in the PBX. You may have ten DID trunks for 100 separate DID telephone numbers. DID trunk circuit boards handle thirty-two, sixteen or eight trunks in most systems. Note: DID trunks delivered on a PRI ISDN circuit may also be used for outgoing calls. This has created the oxymoron two-way DID.

Universal Trunk Circuit Boards

A universal trunk circuit board enables you to mix both-way trunks and DID trunks on the same board. This creates efficiencies in the use of the ports and therefore can lower the cost in terms of the total number of circuit boards to be purchased. It can also save space (sometimes called real estate) within the PBX cabinet.


The trunks that use the above types of circuit boards are sometimes collectively referred to as copper trunks or analog trunks to distinguish them from the outside lines that are delivered on the T-1s, which is common in PBXs today. Copper trunks are still used for back up. The cost of T-1s has dropped dramatically in recent years, so T-1 has become the more prevalent method of delivering the outside lines to the PBX.

T-1 Circuit Boards

A T-1 is a high-capacity circuit using two pairs of wires, enabling the transmission of up to 24 voice conversations at one time. In order for it to do this, a piece of hardware called a multiplexer is required at each end.

The T-1 circuit board is a multiplexer that fits right into the PBX shelf. In some older PBXs the T-1 circuit card must be placed on a shelf specifically designed to handle it. If you are going to use a T-1, you may need to buy a separate shelf for it. Each PBX has limits, so find out how many separate T-1s your PBX can hold. You may hear the term channel bank which is a multiplexer external to the PBX. The channel bank can be used if your PBX does not have T-1 circuit board capability.

PRI/ISDN Circuit Boards

Most PBXs can accept ISDN (Integrated Services Digital Network) lines. The circuit board may be for a PRI (Primary Rate Interface) ISDN line which has 23 B channels plus one D channel. A few PBXs can also accept BRI (Basic Rate Interface) ISDN lines with 2 B channels plus one D channel although the BRI lines are seldom found in a PBX. The ISDN lines can be used for voice conversations and data transmissions.

T-1/PRI Circuit Boards

On newer PBXs, the same circuit board is used for both T-1 and PRI.

Tie-Line Circuit Boards

Tie-lines are point-to-point lines connecting two PBXs so that the users of both systems may communicate without dialing an outside call. Some systems have separate circuit boards for these tie-lines. Most systems use a circuit board which handles two, four or eight tie-lines.

Digital Telephone Circuit Boards

Most telephone systems now use digital telephones. This means that the analog voice signal converts to a digital form right in the telephone and travels back to the PBX cabinet in a digital form (combinations of ones and zeroes). For every 8, 16 or 32 of these telephones in the system it is necessary to have a digital telephone circuit board. This may also be called a digital station board. Each port on the board corresponds to a specific digital desktop telephone in the system. Every telephone has an associated numeric location in the PBX cabinet indicating the port, the circuit card and the shelf.

Analog Telephone Circuit Boards

Most telephone systems are installed with at least one analog circuit board with ports for either 8 or 16 analog telephones. Many single-line telephones are analog. The voice signal is sent from the telephone to the PBX circuit board in an analog form.

The analog ports provide analog extensions required for using fax machines and computer modems through the PBX.

There are differences of opinion as to whether or not faxes and modems are best run through the PBX or through separate outside lines. It is believed that some PBXs may slow down the data transmissions.

Most PBX manufacturers will not guarantee throughput of data beyond a certain speed. If you expect to have computers and fax machines go through the PBX, it is important to find out what speeds can be expected. You may decide to bypass the PBX.

Some systems also use analog ports for interfacing with a Voice Mail or Automated Attendant system although smoother integration is accomplished with a digital interface.

DTMF Circuit Boards

DTMF (standing for Dual-Tone Multi-Frequency, also known as Touch-tone) signaling requires a separate circuit board within the PBX with DTMF receivers. If you have to wait too long for PBX dial tone after you lift your telephone handset, you may need more DTMF receivers. The receiver is engaged when someone within the system is dialing a telephone number and is freed up after the dialing is completed.

Common Control Circuit Boards

These circuit boards house the central processing capability of the PBX. Most PBXs now have common control circuit boards on every shelf, which is a distributed type of processing. Many of the pre-1980's PBXs had the common control on a single shelf only. If it failed, the entire system failed.

How Does The Telephone Work?

Now, let's get to what is inside the telephone. Remember that these components described are commonly found in the older traditional telephones. We are providing this information to lend an historical perspective. Most of today's telephone systems substitute electronic components to emulate these functions.

  1. The transmitter. ( Figure 1) The transmitter is the ear of the telephone in that it "hears" the voice of the person speaking into it. The transmitter is a miniature carbon pile rheostat. A rheostat is a device that controls an electric current by varying the resistance in the circuit, similar to the action of a dimmer switch control. The variations in sound pressure from the voice vibrating against the diaphragm change the compression of the carbon granules. This varies the resistance of the transmitter. The transmitter has two contacts that are insulated from each other. Current can only flow through the carbon granules. As sound pressure from the voice presses against the diaphragm, the carbon is more closely compressed within the chamber. Compressing the carbon granules lowers the resistance of the transmitter resulting in more current flow through the transmitter circuit. When the pressure on the diaphragm is released, it momentarily snaps out farther than its original position. The carbon is under less pressure than normal and the resistance of the transmitter is momentarily greater. The current flow decreases.

    Figure 1: The Transmitter

    The diaphragm of a transmitter is made of lightweight phosphor bronze, duraluminum or a similar material. Either an extra inner cone of the same material strengthens the center or it is corrugated to act as a stabilizer. The flexible outer edge is securely clamped in the transmitter housing. This design enables the diaphragm to move in and out at the center like a piston. Since the diaphragm is sensitive to sound waves, the carbon granules are compressed and released as the corresponding pressure from the sound wave's changes.

    The telephone transmitters in use today are, in principle, like the ones invented more than 100 years ago by Thomas Edison. Many modern electronic telephones use real microphones connected to related speech processing equipment to vary the line current. Small microchips allow economy and space saving, enabling inexpensive, high quality "throwaway" telephones. The output now generated by microchip-based telephones must emulate the same variations created by the carbon granule type of transmitter.

    What is known as the basic 500 set, a single-line telephone like the one that was in use in most homes, has dictated the industry's electrical standard for the telephone instrument and all related signal processing equipment.

    All types of 2- and 4-wire circuits are still designed around that 500 set.

  2. The receiver { Figure 2). The receiver is the "mouth" of the telephone in that it speaks into the ear of the person using the telephone. It also contains a diaphragm whose movement is caused by the strengthening and weakening of the field created by the magnet within the receiver. The receiver converts the varying electrical current representing the transmitted speech signal to variations in air pressure perceived as sound by the human ear. An electromagnetic receiver consists of coils of many turns of fine wire wound on permanently magnetized soft iron cores that drive an armature. The armature is a diaphragm made of a soft iron material.

    Figure 2: The Receiver

    When someone speaks a word into a transmitter, the current flow in the circuit is alternately increased and decreased as the moving electrode moves in and out of the carbon chamber. A requirement for an electromagnetic receiver is a permanent magnet to provide a constant bias field for the varying electromagnetic field to work against. Otherwise, both positive and negative currents would push the armature in the same direction. The varying electrical current representing speech flows through coils and produces a varying electromagnetic field. It alternately aids and opposes the permanent magnetic field; thus, it alternately increases and decreases the total magnetic field acting on the diaphragm. This causes the diaphragm to vibrate in step with the varying current and moves the air to reproduce the original speech that caused the current changes. Other types of receivers operate similarly, except that the armature is a separate part and is connected to a conical non-magnetic diaphragm. The rocking action of the armature causes the aluminum diaphragm to vibrate to reproduce the original speech. In some telephones this receiver is created with the use of microprocessors.

    The electromagnetic receiver was a central element of Alexander Graham Bell's original telephone patent.

    Part of the design of the telephone handset that enables you to hear your own voice while talking is called side tone or side noise. The reason for this is to give you some feedback that the telephone is working. Too much side tone causes an echo.

  3. The ringer. There is a wide variety in types of ringers. Telephones run on DC (direct current) where electrons flow in one direction. The bell or ringer operates on AC (alternating current), which means that electrons are moving in two different directions to activate the bell. This AC sent on the local loop (telephone line) is called ring generator (90 to 105 volts AC at 20-Hz). Minus 48 volts DC is always on the line, which is used to operate the telephone after is answered.

    There is a good analogy for understanding these electrical signals. Envision a garden hose. The hose represents the wire. The water is the current. The water pressure is the voltage (electrical pressure). Stepping on the hose with your foot is equivalent to resistance on an electrical circuit.

  4. Microprocessors. The microprocessors in electronic telephones may replace any of the above internal components and may also add additional capabilities and functions to the telephone, such as speed dialing, etc.

Many telephones look the same, but there is wide variation in the capabilities and prices. You can buy a throwaway single-line telephone for less than ten dollars or a multi-line multi-featured telephone to work with a business telephone system for six hundred dollars. As with many manufactured items, there is variation in the quality of the components that is reflected in the price. The price also tends to be higher on the proprietary telephones, which work with a specific manufacturer's system, even though they may look the same as those you, buy in your neighborhood telephone store.

Parts Of The Telephone

First, we will point out the things you can see, and next we will get to what is inside. Here is a diagram of a typical telephone with its external parts labeled.

Figure 1: Parts of the Telephone
  1. The telephone handset, also called the receiver. In fact, it includes both the receiver enabling you to hear and the transmitter through which you speak. It may also have a volume control or a bar that you can depress to mute either the receiving or transmitting capability. (Buttons on the telephone may also control these functions.)

    Handsets come in different shapes and sizes and are usually made to work with a telephone from a specific manufacturer.

    The handset may be directly wired (also called hardwired) to the telephone cord, which in turn is directly wired into the telephone instrument, or there may be a plastic modular connector at one or both ends of the cord.

    People who spend the entire day on the telephone such as customer service representatives, stock brokerage traders and switchboard attendants often use a headset instead of a handset. On many telephones, you must leave the handset attached and "off the hook" while using a headset, which is cumbersome.

  2. The handset cord. Also known as the curly cord. This often gets very twisted which can break or damage the wires inside causing interference (static or "noise" on the line). Holding it up and letting the handset dangle at the end, enabling the cord to unwind, can straighten it out. As mentioned above, most handsets are connected to the telephone with a small plastic modular connector that plugs into a jack opening on the telephone. Some handsets are hardwired into the telephone and cannot be unplugged.

  3. The mounting cord. A straight cord (cable), usually gray or a translucent gray called silver satin. Typical lengths are 6 feet, 9 feet, 13 feet and 25 feet. This cord sometimes has a modular connector at each end, one plugging into a jack opening on the telephone and the other plugging into a jack opening in the wall. In some cases the mounting cord is wired directly to the telephone or the wall or to both and cannot be unplugged.

  4. The dial pad. Also called the keypad, touch-tone pad, touch-tone buttons or DTMF pad ("DTMF" stands for dual tone multi-frequency, referring to the touch-tone signals). Most telephones use the DTMF method for sending a telephone number to the telecommunications service provider. The local telephone company central office and the business telephone system (PBX) must have the capability to process these tones. The telephone is equipped with the dial pad having 12 buttons that represent the numbers 0 through 9 and the symbols * and #. Pressing one of the buttons causes an electronic circuit to generate two tones. There is a low-frequency tone for each row and a high-frequency tone for each column. Pressing button number 5, for example, generates a 770-Hz tone and a 1,336-Hz tone. By using this dual tone method, only seven tones produce 12 unique combinations. The frequencies and the dial pad layout have been internationally standardized, but the tolerances for variations in frequencies may vary in different countries.

    The touchtone signals are used not only to dial telephone numbers, but also to interact with Voice Processing systems such as Voice Mail, Automated Attendant. (for Sales press 1, for Service press 2, etc.) and Interactive Voice Response.

    Some telephones may have a round rotary dial, but this is becoming less common. The signals sent out by a rotary dial telephone are called dial pulse.

    Rotary dial telephones are still in use. In general, their dial pulses are not recognized by voice processing systems. Some of the newer voice processing systems use voice recognition (Say "Yes" for the Sales Department.)

  5. The feature buttons, also known as feature keys or function keys. These can serve a variety of functions. They enable different outside lines and extensions to be answered. They may activate telephone system functions such as call transfer, call conferencing, call forwarding, etc. They can also be used to speed dial frequently called numbers. Every telephone system manufacturer treats these feature buttons differently, so what you learn about one system may not apply to another. Some feature buttons are flexible, meaning that they can be programmed for a variety of functions. Some are fixed, meaning they can provide only a specific function. Some systems have soft keys meaning that the same button performs different functions at different times.

  6. The display, also known as the LCD (liquid crystal display). Not all telephones have displays, although most newer ones do. All business telephone system manufacturers provide them, but the display telephones may cost more. In most cases they are worth the investment since operating the telephone on a business telephone system without the benefit of the information provided in the display can be cumbersome. Different systems provide different information in the display. Some show the date and time when the telephone is not in use. Some provide instruction prompts to the person attempting to use system functions. Most show the name or extension number of the person calling you, if the call is coming from someone else within your office. Some show the name or telephone number of the person calling you from another location (known as Caller ID or ANI - automatic number identification - if this information is being delivered to your PBX over your outside lines.). A few systems enable you to leave a pre-selected message so that when someone calls your telephone from within your office, his display will read that you are "out to lunch" or "in a meeting." Other systems enable a secretary to send a silent message to the boss while the boss is on another call, although this is not common. As with the feature buttons, the important thing to remember is that telephones and telephone systems from different manufacturers use the display differently. No two are exactly the same.

  7. Lights (also called lamps or LEDs - light emitting diodes) On some systems there may not be a light, but an LCD (liquid crystal display) indicator instead. Most people find the lights easier to see than the LCD. The purpose of the light or LCD is to indicate the status of a call in progress on one of the outside lines or extensions. The light may be red, green, white or amber and more than one color may be lit at the same time. This differs considerably depending upon the manufacturer and model of the system. A light flashing on and off slowly may indicate a new incoming call and is sometimes accompanied by an audible ring. If the same extension appears on more than one telephone it may simply flash at some telephones and audibly ring on others. A steady light usually indicates that the line is in use on either your telephone or another telephone that picks up the same line. A rhythmically flickering light may indicate that a call is on hold. Some telephones have a light next to the button of the extension number that remains lit even when the telephone is not in use.

  8. The switchhook. This refers to those two little plastic buttons that press down on a conventional telephone when you hang up the receiver. When you hang up you are actually breaking an electrical circuit that connected you to the person at the other end while you were talking. On some telephones, the switchhook may be a single bar that depresses when you hang up. Other telephones have a magnetic switchhook inside the telephone, directly under the receiver when it is hung up, that cannot be seen from the outside. In very old movies, we often see someone frantically tapping the switchhook trying to get help as the intruder is banging on the front door. This method was once used to reach an "operator" or switchboard attendant. Instead you would now dial 0. In telephone systems introduced in the 1960's and 1970's, the switchhook was used as a means of activating the system functions such as call transfer. If you held the switchhook down for a second too long, you'd disconnect the call! These functions are now accomplished more easily with feature buttons.

  9. Speaker. Most multi-line and a few single-line telephones are equipped with some type of speaker. A speakerphone enables the person using the telephone to have hands-free conversation with another person at a distant location without lifting the handset. Some speakers are one way only. This may be called a monitor rather than a speakerphone, which is two-way. The monitor enables the person using the telephone to dial out or wait on hold without lifting the handset. They can hear what is on the open line, but cannot speak back to the caller without picking up the handset. Another capability, also called monitoring, enables others in the room to hear both sides of a telephone call in progress, while the person in the room who is speaking uses the handset and the caller at the other end does not sense that he is on a speaker. This is sometimes used for training staff members on how to handle particular types of calls. Speakers may also be used for internal intercom communication only, where someone in on the same premises can call you and his voice will be projected over the speaker. Some systems will allow you to answer back hands-free while others will not. Not surprisingly, this feature is known as hands free answer back!

  10. Message waiting indicator. If the system is working with a Voice Mail system, this lamp or LCD indicator lets you know that you have a message waiting in your Voice Mailbox. There may also be a message on the display of the telephone such as MW for "message waiting." Any of these may also indicate a message waiting at the reception desk or message desk if there is no Voice Mail. Although this use is less common most telephone systems enable a receptionist to manually activate a message-waiting indicator on any telephone in the system. In some systems, the message waiting indicator is a button that, when pressed, will automatically connect you to the Voice Mail system or receptionist to retrieve your messages.

  11. Base of the telephone; telephone housing. This is generally a molded plastic casing designed to house a specific type of telephone.

  12. Faceplate or Face Layout. Most telephones that work with business telephone systems enable you to print a layout of the front (face) of the telephone including the extension numbers and system features that correspond to each button. This printed layout may slip into place over the buttons and under a clear plastic cover that is often called the faceplate.

The Secondary Market


Many companies buy used or refurbished equipment to cut costs. This market is a great place to buy telephones and equipment if you take a little time to do your homework and browse for the best deals. Why not buy refurbished equipment if it can run as good as new, comes with a warranty and saves you money?

Secondary equipment usually costs 30% to 70% less than new. Most vendors will tell you that the older the equipment, the greater the discount. Whatever is scarce costs more money. Phone color also affects value; unpopular colors are scarcer, so cost more.

You can shop for the system you want through the manufacturer or dealer, then call a secondary vendor to see if you can get it for less. Make sure you know your product. The telephone and KSU model numbers indicate analog vs. digital, plus non-visible features like speakerphone.

If you decide to buy from the secondary market rather than a dealer or OEM (Original Equipment Manufacturer), make sure you can install it yourself or that you have someone lined up to install it. Secondary market sources are usually (but not always, so ask) equipment-only.

Many remarketers lease or rent equipment. They also assist in acquiring financing through third-party leasing companies or from the original equipment manufacturer (OEM).

Refurbished equipment is known for its reliability. Some say it’s because it has already endured the burn-in that makes faulty circuits obvious. Refurbished products should be “near new” with all the OEM-provided accessories, up to date software and (at least) the standard warranty.

Make sure you get a user guide with each phone and programming manuals with the systems.

Many remarketers have increased their warranties from 90 days to up to two years. Find out if they provide advance replacement or if you must first return the defective goods. Find out who has to pay the shipping costs.

Remarketers focus heavily on the resale of parts. They should go out of their way to make sure they have in stock the parts you need. Some even track the average rate of failure for various components and stock their shelves according to the forecasted needs of their customers. Find out what the refurbisher’s normal inventory level is for equipment. If they don’t have well-stocked inventory, they may be less equipped to hook you up with the equipment you need when you need it.

Much of the market for secondary systems comes from companies that don’t want to keep their own telecom inventory. Some vendors put serial barcodes on each item, so its history is always known.

Make sure you understand what you are buying. There are many terms that refer to different states of previously-owned and they are used somewhat interchangeably.

The National Association of Telecommunications Dealers (NATD) defines refurbished equipment classes

As Is

Equipment that is bought or sold with no implied warranties. You should expect any condition from inoperative to good. This equipment may not be complete. Buy at your own risk.

Fair Condition Equipment

This equipment is usually in working condition but looks poor.

Good Condition Product

Equipment that is in working condition and looks good.


Generally defined as being sold by an authorized vendor of the Original Equipment Manufacturer (OEM) carrying the OEM’s standard warranty.

Like New Excellent condition

Under normal conditions could pass as new, (not used) but is not necessarily in the Original Equipment Manufacturer (OEM) packaging.


Refurbished equipment is cleaned, repaired, and/or painted (panels, covers, etc.) to restore the appearance of the product to a like new condition. It is completely tested, repaired and ready for installation.

Factory Refurbished Equipment

Factory refurbished equipment has been returned to the factory and the factory has replaced the plastic, repaired what’s broken, upgraded circuit boards, or has otherwise reconditioned the equipment to near-new.


Reconditionedis not a NATD term but it is usually used as synonymous with refurbished.

Before you buy, get a credit report, call other dealers and industry peers. Call the NATD (National Association of Telecommunications Dealers, 561-266-9440) to see if the dealer is a member. If so, see if they are in good standing.

Place several small orders to test dealers before placing a large order.

Telecom Made Simple

Related Posts with Thumbnails