What is PBX?


Private Branch eXchange

The term PBX was first applied when switchboard operators ran company switchboards by hand. As automated electromechanical and then electronic switching systems gradually began to replace the manual systems, the terms PABX (private automatic branch exchange) and PMBX (private manual branch exchange) were used to differentiate them. Solid state digital systems were sometimes referred to as EPABXs (electronic private automatic branch exchange). Now, the term PBX is by far the most widely recognized. The acronym is now applied to all types of complex, in-house telephony switching systems, even if they are not private, branches, or exchanging anything.

An IP PBX is a PBX that provides audio, video, and instant messaging communication through the TCP/IP protocol stacks for its internal network and interconnects its internal network with the Public Switched Telephone Network (PSTN) for telephony communication.

VoIP (Voice over Internet Protocol) gateways can be combined with traditional PBX functionality enabling businesses to use their managed intranet to help reduce long distance expenses, enjoy the benefits of a single network for voice and data and advanced CTI features or be used on a pure IP system which in most cases give greater cost savings, greater mobility, and increased redundancy.

An IP-PBX can exist as a hardware object, or virtually, as a software system. Well known software PBX systems are Asterisk and 3CX. This software can run on PC, performing all functions of casual hardware PBX.

As a phone system, PBX can connect simultaneously to different subsystems and gateways. E.g. It can connect to another PBX, to PSTN by means of VoIP gateway or through third party VoIP provider. Also it can connect to a number of IP Phones and software IP Phones, called Softphones. 

Hardware PBXs

Hardware PBX is electronic device, that realises all the functions of common telepone exchange for local needs (office/business) and gives internal users an opportunity to make outgoing calls to/receive incoming calls from external phone systems.

Configuration of harware PBX is commonly made via special interface, available on LAN connection to PBX.


Software PBX

Software PBX is a special program and/or service running on PC, which performs all functions of PBX by means of computational power of PC. Generally, software PBXs are configured by means of special configuration utility running on the same PC.


Examples of software PBXs are: Asterisk, FreeSwitch, 3CX, 3Com.

Functionality of PBX

Functionally, the PBX performs four main call processing duties:

  • Establishing connections (circuits) between the telephone sets of two users (e.g. mapping a dialed number to a physical phone, ensuring the phone isn't already busy)
  • Maintaining such connections as long as the users require them (i.e. channelling voice signals between the users)
  • Disconnecting those connections as per the user's requirement
  • Providing information for accounting purposes (e.g. metering calls)

In general, PBX functionality gives full control over phone system. It includes:

- Organization of call routing within system

- Auto attendant

- Voice mail

- Day/night hours to reroute calls to voice mail after workday is over

- Hotlist management

- Interactive voice response system

- Welcoming message

- and many others.

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