A business telephone system is any of a range of a multiline telephone systems typically used in business environments, encompassing systems ranging from small key systems to large scale private branch exchanges.
A business telephone system differs from simply using a telephone with multiple lines in that the lines used are accessible from multiple telephones, or "stations" in the system, and that such a system will often provide additional features related to call handling. Business telephone systems are often broadly classified into "key systems", "hybrid systems", and "private branch exchanges".
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, while a private branch exchange operated in a manner similar to the public telephone system, in the calls were routed to the correct destination by being dialed directly. Technologically, private branch exchanges share lineage with central office telephone systems, and in larger or more complex systems, may rival a central office in capacity and features.
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.
Key systems are primarily defined by their individual line selection buttons for each connected phone line, a feature shared with hybrid systems. New installations of true "key" systems have become less common, as hybrid systems and private branch exchanges of comparable size now have similar costs and greater functionality.
Key systems can be built using three principal architectures: electromechanical shared-control, electronic shared-control, or independent keysets.
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