An interface device for an automobile telephone-set with a data transmission device-includes a switch for selectively connecting voice lines of a radio-transmitter/receiver to one of the telephone-set and the data transmission device, a distinguishing device for distinguishing dial data transmitted from the telephone-set and a control unit for energizing the switch and for connecting the voice lines of the radio transmitter/receiver to the data transmission device upon receiving a redial request from the data transmission device. When the user operates the telephone-set, an off-hook signal is transmitted from the telephone-set to the remote terminal through the radio transmitter/receiver. At this moment, the distinguishing device distinguishes the dial data which is transmitted from the telephone-set. The dial data is memorized in the data transmission device in case of a busy or not answerable situation at the remote terminal. When the redial request is generated by the data transmission device, the control unit transmits the memorized dial data to the radio transmitter in order to call the remote terminal without any manual operation of the telephone-set. Further, the control unit energizes the switch in order to connect the voice lines of the radio transmitter/receiver to the data transmission device. Thus the data transmission device is capable of redialing automatically through the control unit with hands-free operation during the driving automobile.