PURPOSE: To enable use of a line efficiency at all times in response to the change in the kind of call speed by using a control channel provided between a multiplexer and a private branch exchange so as to transmit the call conversion speed thereby applying speed conversion and multiple access and multiplex.

CONSTITUTION: One of multiplexed channels of a digital trunk interface(DTI) 2 is used as a control channel and the communication with the processor provided in a processor(CPU) 34 and the DTI 2 is applied through a control channel. Prior to the connection of call, the DTI 2 sends the attribute information of a call and its conversion speed in case of voice compression to a CPU 34 of a multiplexer(MUX) 3. Then the CPU 34 uses the information to converts the speed into a speed designated by a transcoder(TRC) 32. The voice signal subjected to speed conversion is subjected to multiple access and multiple processing by a multiple access multiplex circuit(HMUX) 33 and sent to a digital line 4. In this case, the CPU 34 of the MUX 3 assigns a channel so that the utilizing efficiency of the digital line 4 is improved highest according to the call speed.