Caleb sez, "My kids really have to scream from the back of my VW bus for me to hear them while we're flying down the highway. With this little hack, we have a nice intercom for them to talk to me with!"
The circuit to make two phones work as an intercom is extremely simple. You can see in the picture below, all you really need is a little bit of spare phone cable (or a spare junction box), a resistor, and a power source. A single 680 ohm resistor should let you use the 12V directly from your car or even a standard 9V battery if you’re wanting to make this portable (in a treehouse for example).
This circuit allows both ends to be picked up and audio to be transmitted either way. If you want it to ring, you’re going to have to come up with a slightly different circuit. However, since you’re not using the black and yellow wires, you could theoretically rig up a circuit with a buzzer that would run off the normal 12V.