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.

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.

Completely overkill for just an intercom, but an analog telephone adapter would let you connect those to a VoIP server (like Asterisk, etc). Could be cool.

For just a hotline/intercom setup, I think a telephone line simulator might do the job, but those seem stupidly expensive!

EDIT: The other reply seems a lot more knowledgeable, ignore this one

Thanks! I was looking into a line simulator, but yeah they're expensive!

I thought about setting something up with an old PBX from work, but they're too new for pulse dialing. I was trying to figure out if I could build something with old modems but never got anywhere with that.