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A telephone is normally associated with a directory number (or in general with SIP: AoR). This number is used for placing calls to this telephone and for displaying the telephone's (user's) identity when placing calls to another party. This number is also used when more than one call appearance is supported due to additional features like call waiting.

A keyset denotes a telephone that is associated with more than one number - this allows a given telephone to act on behalf of different phone numbers (users). Just like with traditional telephony systems, people sometimes refer to lines instead of numbers, hence keyset phones are also referred to as multiline phones. The main line (i.e. the line/directory number for user associated with a given physical telephone) is called primary line, whereas all other lines that can be handled on other phones are denoted as shared lines.

At any given time, one telephone can handle only one call appearance for connecting to the handset, and the same applies to keyset / multiline telephones. Also for keyset telephones, features like call waiting can increase the number of call appearances that can be handled, but on a keyset telephone, these appearances may refer to different lines / numbers / users. The programmable feature keys are used for handling the lines and their respective call appearances, supported by the associated LEDs reflecting the line/call status.

The number of lines that can be shared may depend on the phone model.

This feature can be used for different use cases, e.g. chief / secretary installations.

This feature requires specific support by the softswitch / application server. Depending on the selected system, a specific set of additional features for handling call appearances is available (e.g. placing calls on hold).

Similar features are named Bridged Line Appearance (BLA) and Shared Call Appearance (SCA).

See also / Siehe auch