A Private Branch eXchange (PBX) is a telephone exchange that serves a specific business or office, as opposed to one that a common carrier or telephone company operates, whether it’s for businesses or the general public.

A PBX establishes connections (circuits) between the telephone sets of two users. It also maintains such connections as long as the users require them, and then disconnects those connections as per the users requirement. It can also provide information for accounting purposes, such as metering calls. In addition to these basic functions, PBXs offer many other calling features and capabilities.