[boombox]

OK - so the results of my jury rigged setup.

I tried at various times two AND three D cell batteries in each telephone. Battery negative to ZE and battery positive to C.
Line being a connection between both telephones.
ZE was also connected between both telephones.
MC was at times connected directly to battery positive and at times not connected at all.

The push button to ring the bell works - better in one telephone than in the other but I hope that can be resolved mechanically.

I can only VERY faintly hear anything from the earpieces. So faintly I'm not sure it's not just my imagination. There's plenty of voltage so I don't think the problem can be too little power.

Could it be that the mics or earpieces need replacing? Or have I wired it up wrong?

Many thanks.

P.S. Attached a circuit diagram from the telephone's interior.