PURPOSE: To attain ease of interruption connection or callout by calling a slave extension telephone set used most recently when no busy slave extension telephone set exists among slave extension telephone sets.

CONSTITUTION: When a called extension telephone set B does not reply, a calling extension telephone set A dials a specific number T1 in desiring the interruption connection to a nearby slave extension telephone set being busy. The special number T1 is stored in a register R and sent to a central controller CPU via a CPU interface CIF. The central controller CPU accesses a calling extension number AA via a CPU interface CIF. The central controller CPU accesses the calling extension number AA of a storage circuit MEMO to retrieve whether or not a busy display exists corresponding to slave extension telephone set numbers CC0∼CCn. When no busy display exists corresponding to the slave extension telephone set numbers CC0∼CCn, a slave extension telephone set used most recently is retrieved by referencing the operation end time. Then a path of, e.g., the called extension telephone set B is cut off and a path to a slave extension telephone set C1 is reserved instead and a ringing RGT of a signal source TONE is connected to make a call.