An extension telephone is an additional telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 wired to the same telephone line

Telephone line

A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

 as another. In middle 20th century telephone jargon, the first telephone on a line was a "Main Station" and subsequent ones "Extensions". Such extension phones allow making or receiving calls in different rooms, for example in a home. Some telephones intended for use as extensions have built in intercom

Intercom

An intercom , talkback or doorphone is a stand-alone voice communications system for use within a building or small collection of buildings, functioning independently of the public telephone network. Intercoms are generally mounted permanently in buildings and vehicles...

 features.

A telephone extension is an internal telephone line

Telephone line

A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

 attached to a Private branch exchange (PBX) or Centrex

Centrex

Centrex is a portmanteau of central exchange, a kind of telephone exchange.In the United Kingdom, British Telecom markets this service as FeatureLine .-Use of Centrex:...

 system. The PBX operates much as a community switchboard does for a geographic telephone numbering plan

Telephone numbering plan

A telephone numbering plan is a type of numbering scheme used in telecommunications to allocate telephone numbers to subscribers and to route telephone calls in a telephone network. A closed numbering plan, such as found in North America, imposes a fixed total length to numbers...

 and allows multiple lines inside the office to connect without each phone requiring a separate outside line. In these systems, a dialer usually has to dial a number (typically 9 in North America, 0 in Europe) to tell the PBX to connect with a landline (also called DDCO, or Direct Dial Central Office) to dial an external number. Within the PBX, the user merely dials the extension number of the person. Each phone line may be extended up to a fixed maximum.

Reasons to use extensions

Since not all users dial out at the same time, sharing trunk lines

Trunking

In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of...

 is cheaper than giving each phone its own private landline. Extension to extension calls can be made and kept within the company for security reasons. Extensions make it easier for workers to bring their telephone number

Telephone number

A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...

 with them if they change offices in the building. Extensions allow personalized traffic and Call accounting

Call accounting

A Call Accounting System is a telecommunications software or hardware application that captures, records, and costs telephone usage events. Internationally call accounting systems may be referred to as call logging systems...

data to be captured by the PBX or a phone recorder to be used to optimize resources.