Abstract: A communication control unit for integrated service digital network (ISDN) is disclosed. The ISDN communication control unit includes a data communication adapter for transmitting and receiving data to and from the ISDN and a voice adapter for transmitting and receiving voice data received from the ISDN, through the data communication adapter. The voice adapter includes a memory for storing voice data, a compression and expansion circuit for voice data, a temporary storage circuit for voice data, a monitor unit for voice data, a communication path change-over circuit for changing over a communication path between said temporary storage circuit, said monitor circuit and the ISDN, and a control unit for controlling these. A communication path is changed over by the communication path change-over circuit, to mutually transmit and receive voice data between the temporary storage circuit, the monitor circuit and the ISDN.