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This file is a personal continuation of the PBX entry in the MCI Telecommunications Glossary.

A telephone exchange serving an individual organization and having connections to a public telephone exchange is called a Private Branch Exchange (PBX). The PBX performs a switching function by connecting any extension in the private organization to an outside line. A PBX is actually a private switch that connects a group of telephones within an individual organization. Calls placed outside this individual group are connected to a telephone company's central office switch through trunks. A PBX may be operated by an attendant from the private organization or the switching system may be done automatically. Other terms that are commonly used interchangeably with PBX are: Private Automatic Branch Exchange (PABX), Private Automatic Exchange (PAX), and Computerized Branch Exchange (CBX). Although these terms were originally used to identify specific switch structures, today they are often used as synonyms.

PBXs can use any of three basic switching methods: step-by-step (SxS), Cross-bar (X-bar), and computer controlled, to perform the basic function of switching. However, in addition to detecting calls and establishing a transmission link between two telephones, PBXs can do much more.

The common control, often called a central processing unit (CPU), controls the switching matrix that connects the stat ons and trunks. The switching matrix of a PBX performs the same job as does an operator at a manual switchboard or a common control central office switch. The CPU, however, gets its instructions from the "stored program", which contains directions for activities, such as detecting calls, sending them over the best available route, and recording billing information. These computerized electronic switches are used to perform routine, as well as unique, functions that simply weren't practical or even possible with electromechanical switches.

Just as in the public switched network, PBX switches make connection between instruments, or "key telephone sets". We're all familiar with key telephone sets, whether we know them by name or not. They're the business telephones that have six push-button keys lined up below the dial--a red button marked "hold" and five buttons or lines with flashing lights.

Systems with PBXs and key sets have a great deal of flexability in planning for their needs because they can set up their codes to accomplish the functions needed in their particular situations. In fact, the PBX can be programmed so that each individual extension within a system can take advantage of features applicable to its own business needs.

Some of the features that are availiable with PBXs and key systems are: call transfer, which allows internal or external calls to be transferred from one telephone to any other phone in the system; automatic push-button signaling, which indicates the status of all phones in the system with display lights and buttons; one-way voice paging, which can be answered by dialing the operator from the nearest telephone in the system; camp-on, in which a call made to a busy phone automatically waits until the line is idle; and internal and external conference capabilities, which enables outside callers to conference with several inside users.

Some features automatically handle incoming telephone calls. Automatic call waiting not only holds calls made to a busy extension until the extension is free, but also signals the person being called that a call is waiting and informs the caller that he is on hold. Automatic call forwarding will send calls to employees who are temporarily in locations other than their offices, provided they "inform" the PBX where they can be found. Automatic call distribution automatically send an incoming call to the first extension that's not busy--a useful feature for situations in which any one of a group of persons in the organization can adequately respond to incoming calls. Another example is automatic call back, which allows a caller who reaches a busy line to ask the PBX to return his or her call when the line is free.

Still other features provide services such as night telelphone answering, telephone traffic monitoring, and network or hot-line connection. These examples are but a sample from the features possible with computerized PBXs.

Basically, you call the PBX and you will have to enter a code that can be anywhere from 4 to 6 digits (Note: some PBXs do not require codes). Then you will hear a dial tone. From here you would under normal circumstances dial: 9 + 1 (or 0) + NPA-PRE-SUFF, for long distance dialing or dial 8 for local dialing.

The most common use of the PBX is to call Alliance Teleconferencing, a teleconference service offered by AT&T. To do this dial:

PBX Listing

This is a compilation of some PBX's (Private Branch Exchanges). I have tested them all, so they all work.. Let us keep in touch, and people - Moo..