I'm trying to remote control the doors in my stand-alone garage over two wires. There's three operate switches in the house, and three SPDT position switches on the doors. I think I want to end up with a relay board at each side to reflect the state on the other, and some kind of multiplexer in the middle. Off-the-shelf is somewhat preferable to having an Arduino on each side and having to program them.
There's a chance I can abandon the phone in the garage to pick up a second pair, so each direction could have its own.'
I think it's possible an answer would be, "use a 74LS" chip on each end, and connect the output pins a, b, c to the relay controller, the inputs to pins x, y and z and connect the twisted pair to the drivers on pins P and Q.
That would be perfect, having no software, and I'd be able to build that on a proto board, or know what to look for COTS.