The FF33 was the standard field telephone of the German armed forces during World War 2. Typically, FF33s would be connected to a switchboard via the La and Lb/E line terminals (Leitungsklemmen). If necessitated, then the FF33s could be connected together without a switchboard.
Looking at a schematic of the FF33, one can see two jack sockets, K1 and K2, which are connected to line terminals La and Lb/E. K1 and K2 stand for Klinken 1 and 2.
The sockets K1 and K2 are located on the front of the FF33 under a sliding weather plate (Schliessblech).
The sockets are designed to accept a patch cord (Vermittlungsschnur). Back in the day, a patch cord was included with each FF33, but they are almost always missing today. Occasionally, militaria dealers will sell these cords.
The patch cord can be used to connect two FF33s together as shown below. This connection allows telephone station A (Fernsprechstelle A) to talk with telephone station B (Fernsprechstelle B) via station C.
This connection can be expanded to include more FF33s. In the example below, eight FF33s are connected to form stations A, B, C, D, and E.
There are, however, problems with this setup. Stations A, B, C, D, and E are connected together to form a party line. For example, if a person at staton A cranks the magneto on the station’s FF33, the FF33s at stations B, C, D, and E will ring. Therefore, each station must be assigned a ringing code. For example, station A is one ring, station B is two rings, station C is three rings, and so forth. When a person at a station hears his ring code, he knows to pick up the handset and begin talking.
Another problem with the party line setup is that there is no privacy. For example, if a person at station A calls a person at station D, persons at stations B, C, and E can pick up their handsets and listen in on the conversation.
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WW2 German Field Telephone Equipment 