The phone was intended to be technically manipulated only by authorized personnel of the German Postal Service therefore in first sight it does not have any exposed screws or possible means to remove the upper cover. However, there are 5 small white seals labelled with a 'D' that can be punctured and removed by using any sharp object.

Behind them 5 screws allow to remove the main cover as well as the network and receiver's wire cover. Exposing the inner part of the device:

The circuit seems to be very simple and it contains only a handful of electronic components. A quick internet search delivers us a rather low resolution circuit-diagram of the telephone:

The most meaningful components can be therefore straightforward identified and mapped to their physical locations. From the black yellow-topped diodes of the 80s to the tiny isolation transformer on the corner. Although the old electronics have an unique charm, we are only interested in the human-interface peripherals. Namely:

• The rotary dial (more on its operation later)
• The connections to the receiver (microphone and speaker)
• The hook-switch
• The ringer (more on this also later)