An extension telephone is an additional telephone wired to the same telephone line 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 features.

A telephone extension is an internal telephone line attached to a Private branch exchange (PBX) or Centrex system. The PBX operates much as a community switchboard does for a geographic telephone numbering plan 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) to tell the PBX to connect with a landline (also called DDCO) 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.

[edit] Reasons to use extensions

Since not all users dial out at the same time, sharing trunk lines is cheaper than giving each phone its own private landline.^[1] 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 with them if they change offices in the building. Extensions allow personalized traffic and Call accounting data to be captured by the PBX or a phone recorder to be used to optimize resources. [1]

[edit] References

1. ^ Newton, H: "Newton's telecom dictionary", page 901. CMP books, 2004