Each FF33 field telephone has a small white test button (Prüftaste) next to the La and Lb/E telephone wire terminals. This button is used to test the ringer in the field telephone by using the crank magneto. Normally, the magneto rings connected field telephones, but the test button allows the magneto to ring the bells in its telephone.
Normally, the magneto (Induktor) is shorted out by a contact on the magneto. Ringing current from another connected field telephone passes into the La terminal, through the shorted magneto, through the ringer (Wecker), and out through the Lb/E terminal.
If a wire is placed between the La and Lb/E terminals, the test button is pushed, and the magneto is cranked, a loop circuit is created. By turning the magneto crank, a contact closes which connects the magneto into the circuit. When the test button is pushed, its contact PT opens, completing the circuit. Electrical current flows from the magneto, through the ringer, through the Lb/E terminal, into the La terminal, and back into the magneto. The ringer rings.
Here’s how it’s done…
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WW2 German Field Telephone Equipment 