This is a very small telephone exchange system with two ordinary telephones attached. Full bi-directional facility with an intercom system: when a phone is in off hook condition , the circuit will detect it & send ringing pulse to the other phone. When it will be picked up , the connection will be established between both phones. This will take place vice-versa also. This project sounds great if you need a simple and efficient communication system, like in a tree house, you place on telephone in the tree house and the other one in your house. This is just an example but there are far more applications for this system.
Two way telephone system: [Link]