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. Because they incorporate telephones, fax machines, modems, and more, the general term "extension" is used to refer to any end point on the branch. Functionally, the PBX performs four main call processing duties:
- Establishing connections (circuits) between the telephone sets of two users (e.g. mapping a dialled 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)
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 (manufacturers may have a different name for each capability):
- Auto attendant
- Auto dialing
- Automatic call distributor
- Automated directory services (where callers can be routed to a given employee by keying or speaking the letters of the employee's name)
- 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) (the ability to access internal features from an outside telephone line)
- Do not disturb (DND)
- Follow-me, also known as find-me: Determines the routing of incoming calls. The exchange is configured with a list of numbers for a person. When a call is received for that person, the exchange routes it to each number on the list in turn until either the call is answered or the list is exhausted (at which point the call may be routed to a voice mail system).
- Interactive voice response
- Music on hold
- Night service
- Shared message boxes (where a department can have a shared voicemail box)
- Voice mail