This schematic use an old phones as an intercom. You may wish to use a 12 to 24V battery, like a Gel Cell, since the car power or cheap power wall wart style supply is probably too noisy.
You may have to adjust the resistor value (1.2K for 24V). Whatever gets the phones working and does not use more than 30mA total loop current.
If you can manage it, get two speakerphone-type phones. Leave them in "speakerphone" mode continuously and pound on the keypad to attract the attention of the other end.
Source : epanorama.net