PURPOSE: To ensure an effective use of the idle time of the call and thus to realize a big reduction for the channel number by partitioning the call signal into the sound element blocks with every fixed micro-time, detecting the presence or absence of the aural signal every block and then having the channel allotment based on the result of detection of the aural signal for transmission.

CONSTITUTION: Voice detection device 12 connected to multiplication device 11 at the transmission side partitions the PCM-changed call signal into the sound element blocks every fixed micro-time and then detects the presence or absence of the aural signal every block. Based on the detection result, channel allotment 13 is given to telephone circuits 11∼1l with every sound element block for carrying out transmission 15. While at the reception side, the transmission signal is isolated at allotment signal reception device 17, and channel switch 19 is controlled via channel distribution control unit 18 to transmit the transmission signal to analog telephone circuits 22∼2l via PCM decoding multiplication isolator 21. In such way, the idle time of the call can be utilized effectively, thus realizing a reduction of the necessary channel number for the output transmision line.