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.
PBXs are differentiated from "key systems" in that users of key systems manually select their own outgoing lines, while PABXs select the outgoing line automatically. Hybrid systems combine features of both.
Current Trends
One of the latest trends in PBX development is the VoIP PBX, also known as an IP-PBX or IPBX, which uses the Internet Protocol to carry calls. Most modern PBXs support VoIP. ISDN PBX systems also replaced some traditional PBXs in the 1990s, as ISDN offers features such as conference calling, call forwarding, and programmable caller ID. However, recent open source projects combined with cheap modern hardware are sharply reducing the cost of PBX ownership.
Even though VoIP gets a great deal of press, the old circuit switched network is alive and well, and the already bought PBX's are very competitive in services with modern IP Centrexes. Currently, there are four distinct scenarios in use:
* PBX (Private and Circuit Switched)
* Hosted/Virtual PBX (Hosted and Circuit Switched) or traditional Centrex
* IP PBX (Private and Packet Switched)
* IP Centrex or Hosted/Virtual IP (Hosted and Packet Switched)
Since in reality people want to call from the IP side to the circuit switched PSTN (SS7/ISUP), the hosted solutions usually have to maneuver in both realms in one way or another. The distinctions are seldom visible to the end user.
PBX Functions
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)
* Disconnecting those connections as per the users requirement
* Providing information for accounting purposes (e.g. metering calls)
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
* Interactive voice response
* Music on hold
* Night service
* Shared message boxes (where a department can have a shared voicemail box)
* Voice mail
* Voice message broadcasting
* Voice paging (PA system)
* Welcome Message