Understanding PBXs

A private branch exchange, or PBX, is the foundation for most corporate voice networks.
Typically, a corporate telephone network is different from a residential phone system. In a
corporate environment, the network has to serve multiple users who need some advanced
features, such as caller ID, call transfer, and call forwarding. In addition, the typical
corporation would like for its phone system to act like a single network despite the fact that
it serves offices in New York, Raleigh, and London.
Residential telephone systems must allocate a separate external phone line for every user.
The PBX, on the other hand, allows corporate users to share a limited number of external
telephone lines, providing cost savings to the company. It also supports traditional
telephone features like call waiting, call conferencing, and call forwarding. Many larger
corporations connect PBXs together with “tie lines,” which allow corporate users to make
calls to co-workers without placing the call on the PSTN at all. To dial up a user over a tie
line, you typically dial a different phone number, based on the tie line extension.
In VoIP systems, an “IP PBX” is analogous to the PBX of the PSTN, providing many of the
same functions and features as a traditional PBX.