ExtensionAlso known as PBX extension, SIP extension

How SIP extensions work

On an IP PBX, each extension is a SIP account with a username (often the extension number) and a password. A desk phone, softphone or WebRTC client registers with those credentials, and the PBX then knows where to ring it. One extension can have several registered devices, so a desk phone and a mobile app ring together.

• User extensions: people and their devices.
• Feature codes: voicemail, call pickup, parking (for example *97).
• Virtual extensions: ring groups, queues, conference rooms and IVRs.

Extensions and external numbers

A DID can map one-to-one to an extension for direct dial, or many DIDs can point to a main IVR. For outbound calls, the PBX chooses a trunk and caller ID (DOD) based on the extension and dialled number.

Plan numbering early: a consistent length (three or four digits) avoids clashes with feature codes and makes multi-site dialling easier.

Extensions and mobile gateways

Extensions can also be spread across locations. In a multi-site deployment each office keeps its own extension range, for example 1xx in one city and 2xx in another, so staff dial colleagues anywhere by a short number while calls travel between sites over IP.

With a GSM gateway connected as a trunk, inbound calls to a SIM can ring any extension, and extensions can dial out over mobile lines. See GSM gateway for FreePBX for a configuration example.