Glossary · Phone system (PBX)
Extension
In one sentence
An extension is a short internal number, usually three or four digits, given to a user or device on a business phone system. Colleagues dial it to reach each other, and the system uses it to route calls.
Also known as : extension number · extension numbers · phone extension · phone extensions · internal extension · remote extensions
What is an extension?
In phone system language, an extension isn’t just the handset on the desk. It is first of all a short internal number, 112 or 2045, assigned to a user or a location (reception, the meeting room). Colleagues dial it to reach each other, and the PBX uses it to route, transfer and group calls.
On an IP or cloud phone system, an extension is an account: a username, SIP credentials and settings such as forwarding, voicemail and permission to call abroad. Because it is an account rather than a socket in the wall, it follows the person: the same extension works in the office, at home or on the road.
How it works
- Allocation: each user gets an extension in the company’s numbering plan.
- Devices: the extension registers on an IP desk phone, a softphone, a webphone or a mobile app. Several devices can ring for the same extension.
- External number: you can link a DID so customers call the extension directly. Without one, calls come through the main number and reception.
- Settings: the extension carries the user’s preferences: forwarding, voicemail, presence status, ring group and queue membership.
A clear numbering plan helps everyone: 1xx for management, 2xx for sales, 3xx for support, for example. It also makes a call transfer quicker, because numbers are predictable.
Example
A software reseller with offices in London and Edinburgh, 45 people in total, uses four-digit extensions: 1xxx in London, 2xxx in Edinburgh. Account managers each have a direct 020 or 0131 number; the warehouse team only has internal extensions. When an account manager works from home, the same extension rings on their laptop softphone. Colleagues dial 1042 as usual, and customers calling the direct number notice nothing different. When someone moves from London to Edinburgh, IT changes the extension in the portal in minutes instead of re-patching a desk phone.
Extensions at Universal Telecom
On our virtual PBX, each user has an extension that works on IP desk phones, the webphone and the mobile app, with their own forwarding rules and voicemail. Direct numbers can be added through global numbering, including in other countries if your team serves customers abroad.
Frequently asked questions
What is the difference between an extension and a DID?
An extension is internal and only reachable from inside the phone system. A DID is a full public number that rings that extension directly from outside. An extension can have a DID or not.
Does an extension have to be a desk phone?
No. On an IP or cloud phone system an extension is an account that can be used on an IP desk phone, a softphone, a mobile app or a webphone, sometimes on several devices at once.
How do I dial an extension from outside the office?
Call the main number and enter the extension when the menu invites you, or, if the extension has its own DID, dial that number directly.
Related terms
- PBX
A PBX (private branch exchange) is a business's own telephone switch. It connects internal extensions to each other and to the public network, and runs features such as IVR menus, queues and voicemail.
- DID (direct inward dialling)
A DID (direct inward dialling) number, called a DDI in the UK, is a public phone number that rings a specific extension, team or call flow directly, without passing through a switchboard.
- Softphone
A softphone is a phone application that runs on a computer or smartphone. It replaces the desk phone and connects to the company phone system over VoIP, using the business number.
- Call transfer
A call transfer passes a call that has already been answered to another extension or number, either straight away (blind transfer) or after briefing the colleague first (warm or attended transfer).
- Ring group
A ring group (or hunt group) rings several extensions for the same number, either all at once or one after another, so the first available colleague can pick up the call.
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