Away from the front lines, a bell was often connected to the switchboard to audibly alert the operator to incoming calls. The bell was powered by two T30 1.5 volt batteries connected in series. The small 10-line switchboard, for example, has an electrical contact mechanically connected to the flap coils of the 10-lines. The contact is connected to three terminals on the top right of the switchboard. These are labeled “W,” “WB,” and “.” When there is an incoming call on any of the 10-lines, the contact closes. If the batteries and the bell are connected to the terminals, the bell rings.
For calls to outside lines, the rotary dial Amtszusatz is also equipped with three bell terminals. If there is an incoming call on the outside line, and the batteries and bell are connected to the three terminals, then the bell rings.
For a diagram showing how to connect the switchboard and Amtszusatz to batteries and the bell, refer to my WW2 German field telephone equipment book…
WW2 German Field Telephone Equipment 