PURPOSE: To lighten a burden imposed on a controller, to shorten a useless standby time and to improve the efficiency of a function by making a range of trunk, which is selected in accordance with the prescribed priority and the combination of an outgoing terminal and calling time band, variable.

CONSTITUTION: N-number of electronic key telephones EKT1a∼1n are connected to a channel network SW3 through line circuits LC2a∼2n. The SW3 is connected to, for instance, m-group of channel groups LaLm classified by the fee system through m-group of trunk groups TRK4a∼4n. The SW3 and TRK4a∼4m are jointed to a central processing unit CC6 through a control data bus line 5, and controlled so that a channel between the La∼Lm and the EKT1a∼1n and that between the EKT1a∼1n can be formed in accordance with data stored in a memory MEM7 beforehand. With this constitution, ranges of trunks selected according to the prescribed priority can be set variable at an outgoing station and at a calling time band.