The Festungsfernsprecher R (Fortress Telephone R) was one of several types of OB telephones designed to be installed in fortresses and bunkers. The Festungsfernsprecher R was uniquely round in shape and was typically installed near the floor of gun emplacements.
The R operated off of three T30 batteries connected in series to provide 4.5 volts. The batteries were housed in a wall-mounted box, separate from the R. Like other OB telephones, each R contained a crank magneto… The folding crank being situated on the top of the phone.
Also on top of the R was a alarm button and a two-prong socket for a light fixture (Beleuchfung). On the side of the R, there was a standard socket for plugging in a 5-prong plug (5-pol Heeresstecker) of a headphone/microphone set.
Next to the alarm button was a small tag which read, “Bereitisch – Alarm, Taste Drücken Und Kurbel Drehen.” Translated, this means, “Ready – Alarm, Press Button And Turn Crank.” More about this later.
On the bottom of the R is a 14-conductor cable for battery power, phone lines, the alarm circuit, the the lighting circuit.
A circuit diagram (Stromlausplan) for the R is as follows…
The R operates like any other OB telephone, except when the alarm button is pressed while turning the magneto (Ind.) crank. When the button is pressed, its contacts (T I and T II) disconnect the telephone lines La (1) and Lb (2) from the magneto. At the same time, the button contacts connect the magneto to the alarm circuit (11 and 12).
Each room of the fortress or bunker has an electric bell (Wecker). The bells are wired into the alarm circuit. When the magneto in the R is connected to the alarm circuit, and the magneto crank is turned, the bells will sound. The bells alert bunker personnel to move to battle stations.
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WW2 German Field Telephone Equipment 