A brief introduction to PBX theory


Various types of PBX

To understand exactly what a 'PBX' is, it is useful to know a little bit about the history of them.

A 'PBX' was traditionally called a 'switchboard'. The earliest switchboards in companies (or hotels, hostpitals etc.) used an operator (almost always a woman). The operator would physically connect the jack of one extension to the socket of another extension, or to an outside line (the PSTN). See the image on the right to see how that looked. This was a PMBX - a Private Manual Branch Exchange. Exactly the same system was used at public telephone exchanges - think of old films where someone picks up the telephone receiver and says "Operator, get me Kansas 5321!".

Later the Automatic Exchange (electromechanical) together with the rotary dial telephone was invented; this allowed phone users to dial a destination phone number themselves and the call to be routed automatically without the assistance of an operator. Within a company, this was a PABX or PBX (Private Automatic Branch Exchange). In these systems, every phone call was passed between the two parties and across the public network on a physical pair or wires. If you would like to learn more about the history of Electromechanical Telephone Exchanges, Click Here.


With Voice-over-IP, you'll know that your voice and the dialling information is all transferred digitally across your network and the Internet, however the concept of a switchboard (exchange) is still vital. It is still the job of an exchange to keep track of subscribers (location and status) and route calls to and from them. Of course, as PBXs (and public exchanges) developed, many additional services were introduced, such as caller ID, call diversion and call waiting - all of these are managed and operated by the exchange/PBX.

Another important change with IP PBX's is that locally, within your premises, you can use IP phones - the call is carried over your existing data network all the way to your phone, so the phone plugs into your standard RJ45 socket on the wall (you can use WiFi too). With a traditional PBX, every extension had to be wired directly back to the PBX (see the thick cables on the PBX to the right) whereas with an IP PBX, you have just one network cable from the PBX into your existing Ethernet switch, and another network cable to your Internet connection. Obviously, you may have more connections if you choose a hybrid PBX which combines IP and traditional phone/lines connections.

Summary of PBX functions

In summary, a PBX (whether IP, ISDN or analogue, can perform the following:

  • Monitor the Status of a subscriber (busy, absent, free)
  • Time and log all calls made (for statistical or billing purposes)
  • Take voicemail (answerphone) messages in case of busy/no-answer
  • Store the voicemail for telephone retrieval or forward the message by email
  • Route calls from extensions appropriately (to the right destination or interface)
  • Accept incoming calls and pass them to the appropriate extension
  • Transfer calls between extensions
  • Conference Calls
  • Call Waiting
  • Caller ID