I just had a play with a notepad and pencil drawing up some sketches, and I think it can only work as follows:
- When the handset is on the rest, the buzzer is connected across the line to the other phone.
- When the handset is lifted, the battery, microphone and earphone in series are connected across the line to the other phone. Current flows from your battery, through your microphone and earphone, and their buzzer. The trembler contacts are interrupting the current through your earphone, so you hear a sound of the same frequency.
- When the other person picks up their handset, the buzzer is disconnected and.their battery, microphone and earphone are placed in series with yours.
Note that if the batteries are wired the same way around in each unit, the line wires must be crossed over; if the batteries are wired the same way around and the line wires are connected like-to-like, the batteries' voltages will cancel out rather than adding together. This will result in buzzer action, but no speech.
The first thing to do probably is to use an ohmmeter to check for continuity through the handsets, buzzers and switch contacts. (The buzzer
is actually a relay, although it's wired in an unusual way: it has a normally-closed contact in series with its coil and when energised, it switches itself on and off repeatedly at a frequency which you hope is within the sensitive range of human hearing. You should see the resistance reading go from a few tens of ohms to overload, if you operate the contact by hand.)
The fidelity of these units is not great, and anything less than perfect connections throughout will make it worse; but they were a nice novelty back in the days when we were all more easily amused .....