 A key telephone system was originally distinguished from a private branch exchange 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.