OB (Ortsbatterie Betrieb) telephones have onboard crank magnetos and batteries. The batteries power the microphone circuits. The magnetos power the ringers on connected phones.
ZB (Zentralbatterie Betrieb) do not have onboard magnetos or batteries. The microphones are powered by a battery in a central exchange. The ringers are powered by an alternating current source in the central exchange. The modern civilian telephone system is a ZB system.
The differences between an OB and ZB telephone is basically illustrated in the circuit diagrams below.
LK = Loud Speaker Button (Lauthörknopf)
I = Voice Transformer (Sprechspule)
M = Microphone
F = Speaker (Fernhörer)
B = Battery
U = Switch Hook
W = Ringer (Wecker)
Ind = Crank Magneto (Induktor)
C = Capacitor
Although it is not shown above in the ZB circuit diagram (Schaltung) , ZB telephones can have a dialer circuit for automatically activating dialing mechanisms (e.g. Strowger Switches) in the central exchange.
OB telephones wont work on ZB networks. During World War 2, the German military used two pieces of equipment to get around this problem: the Amtsanschließer 33 and the Amtszusatz.
The Amtsanschließer 33 plugs in to the OB FF33 field telephone with a patch cord (Vermittlungsschnur). The Amtsanschließer 33 has terminals to connect it to the La and Lb wires of a ZB network. The Amtsanschließer 33 has a rotary dial and an internal buzzer to make and receive calls on the ZB network.
The single Amtszusatz was a rotary dial attachment for the Small 10-Line Switchboard. A triple Amtszusatz was a three rotary dial attachment for the Big Switchboard. Both the single and the triple can be connected to a ZB network. Each Amtszusatz has a relay to drop a signal flap (and energize an optional DC bell) when an incoming call comes over the ZB network. OB field telephones are connected to the ZB network via switchboard patch cords and a socket on the Amtszusatz.
For more information about OB and ZB systems, switchboards, the Amtsanschließer 33, and the Amtszusatz, check out my book… https://www.lulu.com/en/us/shop/rotwang-manteuffel/world-war-2-german-field-telephone-equipment-a-basic-guide-for-reenactors-and-historians/paperback/product-976w9q.html?page=1&pageSize=4
WW2 German Field Telephone Equipment 