Intercom Made from Rotory Phones
Note: This is a placeholder gallery, I am going to add more to it, but I need to take some pictures first.
One of these phones sits on my desk, the other is in the bedroom where the wife likes to watch TV. If either of us pick up the receiver both phones emit a beep until the other party picks up their phone. They are wired directly to each other with a 9v battery powering this closed system.