Here’s a HOWTO explaining the best ways to rewire the old phone lines and phones in your house or office to use them as an intercom. I haven’t had a landline I used regularly since 1999, but every place I’ve lived has had tons of landline wiring, and old style phones are cheap like borscht. Once you’ve got this rigged, you can pick up that bat-phone on your desk, ring your loved one in the next room, and bark, “Schweetheart, get me rewrite!”
Talking over the phones is easy. You put DC current through the phone and it transmits and receives audio. So two phones and a current source (about 25mA) all in series will give you a talking circuit. A suitable current source can be as simple as a 9V battery and a series resistor whose value is adjusted (with both phones offhook) till about 25mA flows. You can then bypass the battery and the resistor with a capacitor to couple the audio straight across and get a loud and clear connection.
What is much harder is signaling the other end. To ring the bell you need to put 90V (RMS) 20Hz AC into the phone (nominally). Lower voltages will work (down to about 40V) but different frequencies won’t. You can’t ring the phone at 60Hz. I have a ringing circuit in a PBX I built but it consists of a 20Hz sinewave generator, a push-pull power booster and a big transformer. Much too elaborate for a simple 2-phone intercom circuit, and anyway the ringing voltage could painfully zap a kid.
(via Negatendo0