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.