Should be a field telephone. If you google Feldfernsprecher you get plenty of pics to see how it looked. It is a basic telephone that connects with wires, there are no dials. If you turn a crank the field telephone of your counterpart rings and you can talk to him. For example in the defense you could connect an outpost with the platoon/ company HQ and thus avoid using wireless (if you had any). It was in use in the basic form of WW 2 in the Bundeswehr for decades.
Here you can see it live and in colour : http://www.youtube.com/watch?v=P3ZqpROYhJU