The circuit to make two phones work as an intercom is extremely simple. You can see in the picture, all you really need is a litttle bit of phone cables , one junction box , 680Ohm resiter (even without resiter this circuit will work, to get best result allpy resister) & 12v or 9V DC power source (Battery or Adopter).
This circuit allows both end to be picked up and audio to be transmitted either way. If you want it to ring, you're going to have to come with a slightly different circuit.
V Ramesh Kumar.