PBX (Private Branch Exchange)

A PBX, or Private Branch Exchange, is the phone system that belongs to one organisation. It gives each person an extension, lets extensions call each other without going through the public network, and connects to outside lines so that external calls can come in and go out. Features such as voicemail, call queues, auto attendants, conference rooms and call recording live in the PBX.

Older PBX systems were hardware cabinets wired to analogue or digital phones. A modern IP PBX is software, usually running on Linux, that speaks SIP to phones and softphones and RTP for audio. Asterisk is a widely used open source engine, and distributions such as FreePBX and Issabel package it with a web interface. Commercial and cloud-hosted PBX products do the same job as a service.

To a softphone, the PBX acts as registrar, proxy and often media relay. You configure the PBX address as the server, your extension number as the username and the extension's secret as the password. The PBX then routes your outgoing calls to trunks and delivers incoming calls to whichever devices are registered for your extension.