During World War 2, the Germans had Zentralbatterie Betrieb (ZB) telephone service. There were two types of ZB service: with self-dailing (Wählerbetrieb a.k.a. W-Amt) and without self-dialing (ZB-Amt). The ZB service gateway for the Ortsbatterie Betrieb (OB) field telephone switchboard was the Amtszusatz.
ZB-Amt is provided by exchanges with manual switchboards. To illustrate, if the OB switchboard operator presses the Kellogg Switch on his Amtszusatz to the “Abfragen” position, the Amtszusatz is connected to the ZB exchange. This alerts the ZB switchboard operator at the exchange and he connects with the OB switchboard operator’s Amtszusatz. The OB switchboard operator then requests to be connected with one of the ZB exchange’s lines. The ZB switchboard operator makes the connection. The dial on the Amtszusatz is not used.
W-Amt is provided by self-dialing exchanges operated with automatic line switching devices (e.g. Strowger switches) without a switchboard operator. The OB switchboard operator can press “Abfragen” on his Amtszusatz and dial the telephone number of the line he wishes to connect to. The line is automatically connected without an operator.
Some OB switchboards had two Amtszusatzen: one connected to a manual exchange (ZB-Amt) and one connected to a self-dialing exchange (W-Amt).
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WW2 German Field Telephone Equipment 