Abstract: A transmission control unit for controlling transmission and reception of data between electronic devices via a telephone without any use of acoustic coupling is disclosed. The transmission control unit is equipped with a first terminal for detachably connecting a main body of the telephone, a second terminal for detachably connecting a handset removable from the main body of the telephone, and a third terminal for detachably connecting the electronic device. When the main body, the handset and the electronic device are, respectively, connected to the first, second and third terminals, the transmission control unit permits analog electronic data from the electronic device to be input to the main body of the telephone at the handset interface level, and permits analog electronic data from the main body to be sent to the electonic device.