OB (Ortsbatterie Betrieb ) telephones include field telephones like the Feldferrnsprecher 33, the Feldferrnsprecher 43, and the Tischfernsprecher 38. OB telephones contain a crank magneto to generate ringing current and a battery to power the microphone in the handset.
ZB (Zentralbatterie Betrieb) telephones rely on a centrally located voltage source to power the ringer and the microphone. This voltage source is located in a central office or exchange. Civilian phones are typically ZB phones.
Here’s an OB telephone circuit (Schaltung) compared to the circuit for a ZB telephone:
W = Wecker (Bell), Ind. = Induktor (Magneto), U = Umschalter (Talk Button/Switch Hook), C = Kondensator (Capacitor), M = Mikrofon (Microphone), F = Fernhörer (Speaker), B = Batterie (Battery), and J = Sprechspul (Speaking Coil).
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WW2 German Field Telephone Equipment 