A key system (also known as a key telephone system, or KTS) is a multi-line telephone system in which two or more phones share two or more phone lines. Each phone line the key system station set (or extension) has access to has an appearance on the station set (i.e., the telephone). Typically this takes the form of a back-lit button. If the button is dark, the line is available. If the button is lit, the line is in use. If the button is flashing, the line is on hold.
A larger key system (e.g., a dozen or more lines) will typically feature a key service unit (KSU). This is the device where the lines from the voice provider terminate. This device then extends certain lines to certain phones within the building. In smaller systems, the KSU functions are incorporated into each phone.
The distinction between a key system and a PBX used to be very clear: a PBX provided a switching function and dynamic circuit sharing on the trunk; a key system required each phone line to extend from the station sets all the way back to the voice provider. Modern key systems, however, blur this distinction by adding switching and dynamic circuit sharing, making them almost indistinguishable from a small PBX.
Other features now commonly found in key systems include:
- voice mail
- call accounting
- speed dialing
- caller ID
- remote supervision
- wire reduction (each phone line used to require a hard cable connection to each phone. Today's sets can reduce wiring requirements)
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