BTX modem "DBT-03 modem" for access to the Deutsche Bundespost's BTX network

The modem is the connecting piece between a BTX-capable device and a telephone. The function of the device provided the name—during modulation, BTX signals are sent through the telephone line and received by the computer. Demodulation ensures that the incoming BTX signals are filtered from the telephone network and transmitted to the terminal.
Opening the modem is not allowed because the hardware and port ID are stored in it. A seal should prevent people attempting to do this.