When I was developing the VoIP Fundamentals course, I wanted a demonstration to illustrate the simplicity of the public switched telephone network. After combing through the internet, I came across this ingenious little project. With just a 9V battery, a resistor, a telephone cord, and a couple of phones you can create a simple little intercom system. The video explains it fully.
Like I said in the video, I’m sure you could work out a simple signaling method if you did wanted to use the setup as an honest to goodness telephone. I toyed with the idea of using an Arduino to produce the ringing signal, but that signal would need to be amplified significantly.