PBX

A Private Branch Exchange (PBX) is a telephone exchange that serves a particular business or office, as opposed to one that a common carrier or telephone company operates for many businesses or for the general public. PBXs are also referred to as:

• PABX - private automatic branch exchange
• EPABX - electronic private automatic branch exchange

PBXs make connections among the internal telephones of a private organization — usually a business — and also connect them to the public switched telephone network (PSTN) via trunk lines. They incorporate telephones, fax machines, modems, and more, the general term "extension" is used to refer to any end point on the branch.

Two significant developments during the 1990s led to new types of PBX systems. One was the massive growth of data networks and increased public understanding of packet switching. Companies needed packet switched networks for data, so using them for telephone calls was tempting, and the availability of the Internet as a global delivery system made packet switched communications even more attractive. These factors led to the development of the VoIP PBX. (Technically, nothing was being "exchanged" any more, but the abbreviation PBX was so widely understood that it remained in use.)

The other trend was the idea of focusing on core competence. PBX services had always been hard to arrange for smaller companies, and many companies realized that handling their own telephony was not their core competence. These considerations gave rise to the concept of hosted PBXs. In a hosted setup, PBXs are located at and managed by the telephone service provider, and features and calls are delivered via the Internet.

Functions

Functionally, the PBX performs four main call processing duties:

• Establishing connections (circuits) between the telephone sets of two users
  
• Maintaining such connections as long as the users require them
  
• Disconnecting those connections as per the user's requirement
• Providing information for accounting purposes
  

In addition to these basic functions, PBXs offer many other calling features and capabilities, with different manufacturers providing different features in an effort to differentiate their products. Common capabilities include :

• Auto attendant
• Auto dialing
• Automatic call distributor
• Automated directory services
  
• Automatic ring back
• Call accounting
• Call forwarding on busy or absence
• Call park
• Call pick-up
• Call transfer
• Call waiting
• Camp-on
• Conference call
• Custom greetings
• Customised Abbreviated dialing (Speed Dialing)
• Busy Override
• Direct Inward Dialing
• Direct Inward System Access (DISA)
  
• Do not disturb (DND)
• Follow-me, also known as find-me: Determines the routing of incoming calls. 
• Interactive voice response
• Music on hold
• Night service
• Shared message boxes
  
• Voice mail
• Voice message broadcasting
• Voice paging (PA system)
• Welcome Message