Extension

An extension is the number you dial to reach a colleague inside the same phone system, typically three or four digits such as 201 or 1042. On an IP PBX, each extension is a configuration record that holds the number, a display name, a secret, the allowed codecs, voicemail settings and other options. The Asterisk documentation describes these records as endpoints in PJSIP or peers in the older chan_sip.

When a softphone registers with a PBX, the extension number is normally the SIP username. The address of record becomes sip:201@pbx.example.com, and the PBX routes any call to 201 to whichever devices have registered under that name. Several devices can share one extension; a desk phone and a softphone can both ring.

Because extension numbers are easy to guess, many PBX systems allow a separate authentication name that differs from the extension. In SIPdia the extension goes in the Username field and any separate authentication name goes in Auth username under Advanced.

An extension is different from a provider account, which is a login for an external SIP service rather than a number on your own PBX. The tutorial on that distinction explains how the settings differ.