@XerTheSquirrel I think what you're asking for is a two-way phone line simulator, and phone line simulators are definitely things.
The simpler ones start ringing one end 2 seconds after you take the other end off hook. The more advanced ones are also called FSX phone line simulators and PSTN simulators.
@XerTheSquirrel Ah. Yeah, that's trickier. I can't find anything fitting the bill short of a full-blown PBX system. :/