Extension (internal number)
An extension is a short internal number, such as 101, that reaches a specific person, team or menu inside a company phone system without dialling a full phone number.
How extensions work
Colleagues call each other by dialling the extension, and the call stays inside the phone system. External callers can reach an extension through a menu ("If you know your party's extension, dial it now") or when an operator transfers them. A company can have many extensions behind a single public DID number.
Extensions are often grouped by team, for example 1xx for sales and 2xx for support, so they are easy to remember.
Extensions in phone.systems™
In phone.systems™ extensions are called Internal Numbers. Each user can have an internal number for incoming calls and an internal caller ID that colleagues see. Internal numbers can also point to a call flow, which lets you give a team menu or queue its own short number. The Dial Plan decides where each number goes — a user's app, a SIP device, a call flow or an external route.
A common pattern is to number extensions by team, for example 101 and 102 for Sales, 201 and 202 for Support and 301 for Billing. See the business phone system.