Private branch exchange

A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging from small key telephone systems to large-scale private branch exchanges.

A business telephone system differs from an installation of several telephones with multiple central office (CO) lines in that the CO lines used are directly controllable in key telephone systems from multiple telephone stations, and that such a system often provides additional features related to call handling. Business telephone systems are often broadly classified into key telephone systems, and private branch exchanges, but many hybrid systems exist.

A key telephone system was originally distinguished from a private branch exchange (PBX) in that it did not require an operator or attendant at the switchboard to establish connections between the central office trunks and stations, or between stations. Technologically, private branch exchanges share lineage with central office telephone systems, and in larger or more complex systems, may rival a central office system in capacity and features. With a key telephone system, a station user could control the connections directly using line buttons, which indicated the status of lines with built-in lamps.

Key telephone systems are primarily defined by arrangements with individual line selection buttons for each available telephone line. The earliest systems were known as wiring plans and simply consisted of telephone sets, keys, lamps, and wiring.

Key was a Bell System term of art for a customer-controlled switching system such as the line-buttons on the phones associated with such systems.

The wiring plans evolved into modular hardware building blocks with a variety of functionality and services in the 1A key telephone system developed in the Bell System in the 1930s.

Key systems can be built using three principal architectures: electromechanical shared-control, electronic shared-control, or independent key sets.

New installations of key telephone systems have become less common, as hybrid systems and private branch exchanges of comparable size have similar cost and greater functionality.

Before the advent of large-scale integrated circuits, key systems were typically composed of electromechanical components (relays) as were larger telephone switching systems.

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