I connected self powered crank telephones to my home phone wiring for use as intercoms. Crank one and the others ring. Pick up to engage the battery powered talk circuit and you're chatting with Mabel like it's 1915. I did replace the old dry cell batteries with lithium camera cells so I don't need to think about them for a decade.

We have an Ethernet run that's not in use right now. I had a Cisco ATA from earlier experiments so I picked up 2 old phones with proper bell ringers and now the kids can call up from the living room to my office when it's time for dinner, they love it.