[russell_w_b]

Here's a way that may work as an intercom, although I've yet to try it. Couple of PO3000 relays and a 10u capacitor as a transmission bridge. A third wire is necessary and slave LJUs can be used to terminate, allowing 50Hz a.c. signalling operated by the bridging relays.

Tele A is lifted, RLA energises from a 12V battery. RLA2 opens ensuring no a.c. ringing goes back to Tele A, and RLA1 closes putting 50V a.c. onto the bell of Tele B.

Tele B handset is lifted, energising RLB from 12V battery. RLB1 opens and cuts off ringer a.c. supply. RLB2 closes, but the ringing circuit is held open by RLA2. Speech goes round the loop steered by the transmission bridge formed by the relays and the capacitor.

But I suspect baceplate wishes to use a small exchange and 'house' system?