The Tischfernsprecher 38 (Desk Telephone 38) is a desk version of the FF33 field telephone. It consists of a crank magneto phone and a battery/terminal box (Beikasten). The Tischfernsprecher 38 was typically used in an office environment by higher ranking German officers.
To explain how the Tischfernsprecher 38 works… The telephone can be hooked up to a ZB (Zetralbatterie Betrieb) system or OB (Ortsbatterie Betrieb)/SB (Schlusszeichen Betrieb) systems.
ZB is compatible with the modern automatic central battery civilian telephone system. During WW2, the La/ZB and Lb/E terminals were to connect the phone to the ZB Selbstanschlußanlagen (SA) telephone network. When connected to the SA network, the Tischfernsprecher 38 had no rotary dial, so it could only receive incoming calls.
In OB operation, the La/OB/SB and Lb/E terminals were used to connect the phone to an OB switchboard like the small switchboard for 10 lines. The terminals could also be used to connect the phone to an SB switchboard like the Grosser Fieldklappenshrank with an Untersatz C base unit, The SB system was abandoned mid-war.
Referring to the electrical schematic below… The Tischfernsprecher 38 automatically switches between ZB and OB/SB connections when the handset is place in or removed from the cradle. This is accomplished by switch U, contacts IV and V. When the handset is in place, and the ZB connection is used, ringer voltage is directed through capacitor C1 before going to the the bells W. When the OB/SB connection is used, C1 does not tie in and the ringer voltage a passes directly to the bells W. When the handset is lifted from the cradle, the voice signal passes directly through to the the handset, when the ZB connection is used. When the OB/SB connection is used, and the handset is lifted, the voice signal is passed though capacitor C1 on the way to the handset. The handset microphone is powered by a 1.5 volt battery MB and the speaker is powered by the phone system. Power is supplied via switch U contacts I, II, and III. In other words, when the handset is lifted from the cradle, it turns on. To talk, the handset switch SpT must still be depressed.
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WW2 German Field Telephone Equipment 