Key Telephone Systems

A KTS is equipped with several buttons that allow a caller to directly select outgoing lines or incoming calls, and use intercom and conference facilities. Unlike in a PBX, a user does not have to dial '9' to access an outgoing line, but the incoming calls are not automatically routed to the called party. KTS are user-installable and allow handling of multiple lines without paying a surcharge to the telephone company. However, the hunt-group service cannot be provided on them.

A key system or key telephone system is a multiline telephone system typically used in small office environments. Key was a Bell System term of art for a manually operated switch, such as the line-buttons on the phones associated with such systems. A key system was originally distinguished from a private branch exchange (PBX) in that it allowed the station user to see and control the calls directly, manually, using lighted line buttons. Key systems are noted for their expandability and having individual line selection buttons for each connected phone line. Some features of a private branch exchange such as dialable intercoms may also be present.

The line between the largest key systems and full PBX systems is blurred. In the 1A2 days, the line was clear: 1A2 systems did not allow the sharing of anonymous "trunk" lines and PBX systems did. Modern key systems blur this distinction by often allowing this feature.

The modern key system now supports ISDN, analog handsets (in addition to its own dedicated handsets - usually digital) as well as a raft of features more traditionally found on larger PBX systems. Their support for both analog and digital signalling gives rise to the "Hybrid" designation.

The modern key system is usually fully digital (although analog variants persist) and some systems embrace VOIP. Indeed, key systems now can be considered to have left their humble roots and become small PBXes. Effectively, the aspects that distinguish a PBX from a key system are the amount, scope and complexity of the features and facilities offered.

	Key systems can be built using three principal architectures:
		Electromechanical shared-control
		Electronic shared-control
		Independent keysets