[timonoko]

Connecting two rotary phones (via 40 volt battery) is the easiest thing. They work as-they-are without modifications. When the phone is off-hook, the dialling rotator makes pulses and when the phone is on-hook those pulses cause the bell to ring. But not very loudly, because the real ringing pulses are 80 volts.

Source: I am rotary-telephone-era Telecommunication Engineer.

[drewzero1]

Thanks for the info! Sounds a lot easier than anything I was imagining. I'll have to dig up a couple of those 4-pin sockets and see if I can source a battery.