Glossary Phone system
What is Extension?
Also called ext, internal number, seat
An extension is the short number, typically three or four digits, that identifies a user or device within a phone system. Dialling it from inside connects directly without going out to the public network, which is why internal calls are free and instant.
Extensions and DIDs are different things
An extension is internal; a DID is a public number that reaches it from outside. They are usually paired, and often the last digits match to keep things memorable, but they are separate objects. One extension can have several DIDs pointing at it, and a DID can point somewhere with no extension at all, such as a queue or an auto attendant.
Numbering plans are worth planning
Extension numbering is easy to get wrong and painful to change once it is on business cards and in muscle memory. Leave room to grow, avoid ranges that clash with emergency or outside-line prefixes, and if you have multiple sites, decide early whether site codes are part of the scheme. Retrofitting a numbering plan across a live business is genuinely unpleasant.
Extensions that are not people
It is worth reserving a block of extensions for things rather than staff: conference rooms, a warehouse phone, a lobby handset, an overhead paging device, a fax line. These get forgotten in numbering plans and then squeezed in wherever there is space, which makes the directory harder to reason about later. Giving non-human endpoints their own range keeps the user range clean and makes it obvious at a glance what an unfamiliar extension is when it appears in a call record.