Abstract: The present invention relates to apparatus for use with a telephone system having a plurality of stations each of which becomes active when the station goes off-hook and one of a plurality of keys at the station is actuated for selecting one of a plurality of outside central office (CO) lines connected to the stations through a key service unit (KSU). The apparatus of the present invention comprises a station control unit (SCU) associated with each station for connecting the same to the KSU; and a main control unit (MCU) by which the KSU is connected to the outside CO lines. The MCU comprises a microprocessor operated central control; and a multistate line control unit (LCU) operatively associated with each outside CO line for connecting each station in parallel to the selected CO line and/or to the central control in accordance with the state of the LCU operatively associated with the selected CO line.