Conclusions

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„ „ Telephone System Design (PBX and Key Systems)

The way in which a telephone system is set up is often called the system design. Up until the 1980’s, the differences between Key Systems and PBXs were clear. Key systems were for smaller businesses, and their distinguishing characteristic was that each outside line (outside telephone number) appeared on everyone’s telephone.

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A typical key system for a small office may be set up in the following manner. When the first line is in use, incoming calls ring on the second line and on down through the sequence of telephone numbers. This is known as the hunt group. It contin-ues until reaching the last line in the group. With all lines in use, the caller hears a busy signal.

If the receptionist answers a call, it is put on hold and the called person is told to pick up on line 3. When the button is depressed for the third line at any telephone in the office, the caller is there.

Users of the key system press down a key or button of a line to place an outgoing call. They hear the dial tone coming from the local telephone company central office. There is no need to dial 9 to place an outside call.

Many key systems are still set up in this manner and are re-ferred to as a square key system (Figure 2.2). This means that every telephone looks the same and picks up the same group of outside lines.

This works well in offices where there are no more than ten outside lines. After that, it becomes difficult to remember who is on which line and is confusing for both users and callers left on hold and asked several times, “Who are you holding for?”

Figure 2.2

A Square Key System

INTCM 1 INTCM 2 INTCM 3

If a company has been operating with a square key system with more than ten lines and it is working well, there may be no need to change if you replace the system.

Key systems usually have an intercom so that the reception-ist can announce a waiting call to the person requested by the caller. The intercom also enables system users to call each other.

Key systems typically have more options in terms of internal communications than do PBXs.

You may hear that a telephone system is “just a key system,”

used in a derogatory manner to suggest that it is not as good as a PBX. In fact, most of the key systems currently on the market incorporate many, if not all, of the functions of a PBX, so the distinction has become blurred. You may see the term hybrid referring to a key system that can be set up either like a square key system or a PBX. When selecting a telephone system, it is more important to focus on the system capabilities for features and growth than on whether it is a hybrid key system or a PBX.

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A PBX has traditionally been set up in the following manner.

The organization has a main telephone number, sometimes end-ing in a double or triple zero (for example 635-5300). When this telephone number is busy, the calls roll over to the second line and then on through the hunt group as we described with a square key system.

With a PBX, there may be a greater number of outside lines and these lines do not appear on every telephone. Instead, they are answered by a central position known as the switchboard or attendant console. The caller tells the switchboard operator or attendant the name of the person called. “May I please speak to Linda Storaekre?” Then the operator extends the call to that person by dialing an extension number assigned to the called person’s telephone.

To place an outgoing call, the same group of outside lines is used. Since the person placing the call has only an extension

number on his telephone, it is necessary to press the button for that extension number and dial an access code, usually 9. In doing so, a free outside line is selected and the person can then place a call. If all lines are in use, as with a square key system, callers to the main telephone number hear a busy signal. People trying to place outgoing calls, when dialing 9, will also hear a busy signal (sent from the PBX, not the local telephone com-pany central office) indicating that all lines are in use.

It is important to understand that the total number of tele-phone calls in progress, whether they are incoming or outgoing, cannot exceed the total number of outside lines. If you have ten outside lines and receive four telephone calls at the same time as six people are placing outgoing calls, the next caller to your tele-phone number will hear a busy signal. Outside lines are also called telephone numbers, dial tone lines, trunks and POTS lines (plain old telephone service).

What we have described above, with all calls coming into a switchboard operator, is a traditional telephone system design (Figure 2.3). With the increasing use of Voice Mail and Auto-mated Attendant, many systems are being set up differently. For example, a type of outside line called a Direct Inward Dial trunk may be used. This allows each person using the system to have a separate telephone number to give to callers so that their specific extension may be called directly without the intervention of the switchboard operator. If the directly-dialed extension is not an-swered or is busy, the call may then go to the switchboard operator or it may be answered by Voice Mail. The caller hears the voice of the person called. “This is Christine Kern. I’m not at my desk right now, but please leave a message and I will return your call, or dial 0 for immediate assistance.” The caller may then leave a message that will, in turn, activate a message waiting indicator on the telephone of the person called (Figure 2.4).

An Automated Attendant can take the place of the switch-board operator answering the main telephone number with an announcement such as the following: “Thank you for calling CMP Books. If you know the extension of the person you are calling,

• The Telecom Handbook Traditional Telephone System

Incoming Call – A call comes in on a combination trunk and is answered by the switchboard attendant. Caller is then sent to the correct extension.

Outgoing Call – Extensions dial 9 to get dial tone from the combination trunk for placing an outgoing call. Switchboard attendant is usually not involved in this process.

Combination Trunks For Incoming And Outgoing Calls

Telephone Extensions Local Telephone Co.

Central Office

Switchboard Attendant

Console PBX

Figure 2.3

The Telecom Handbook • 39 CHAPTER 2

Telephone System With “DID” Trunks and Voice Mail

Incoming Call – In this scenario, calls are placed directly to the telephone extension, bypassing the switchboard attendant.

When the extension is unanswered or in use, the calls forward to Voice Mail.

Outgoing Call – Extension dials “9” and selects a combination trunk.

Combination Trunks For Outgoing Calls Only

“DID” Trunks For Incoming Calls Only. Every Extension Can Be Dialed Directly

From The Outside.

Telephone Extensions Local Telephone Co.

Central Office

PBX

VOICE MAIL VOICE MAIL VOICE MAIL VOICE MAIL VOICE MAIL

“This is Beth Cockrell.

I’m not at my desk right now. Please leave

me a message and I’ll call you back.”

• The Telecom Handbook Telephone

Answered

Telephone Unanswered

Telephone System With Automated Attendant and Voice Mail

Incoming Call – The PBX, Automated Attendant and Voice Mail are housed in the same room and sometimes within the same cabinet. The call movement is controlled by the system software. Callers answered by the Automated Attendant direct their calls to the correct extension.

Outgoing Call – Extensions dial “9” to place outgoing calls on combination trunks.

“This is Rich Laino. I’m not at my desk right now. Please leave me a message and I’ll call you back.”

“Thank you for calling. If you know the extension number you may dial

it now. For sales, press 1…”

VOICE MAIL VOICE MAILVOICE MAIL VOICE MAILVOICE MAIL

AUT AUT AUT AUT AUTOMAOMAOMAOMATEDOMATEDTEDTEDTED

ATTEND ATTENDATTEND ATTENDATTENDANTANTANTANTANT

PBX Combination Trunks For

Incoming And Outgoing Calls

Local Telephone Co.

Central Office

please dial it now. To order a book, dial 1; to place advertising, dial 2; for a company directory, dial 3; or please wait for assis-tance.” You still need a live person at some point to handle callers who do not know what they want or who do not wish to use the automated system (Figure 2.5).

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