Extension (telephone)

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A telephone extension is an interoffice telephone line attached to a 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 to dial an external number. Within the intra-office line the user merely dials the extension number of the person. Each phone line may be extended up to a fixed maximum. Since not everybody uses the phone at the same time, it is cheaper for the office to install the PBX than to have each phone have its own private landline.

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