[majoconz]

If you can hunt out an old booklet called "Out of Thin Air" published by the old Practical Wireless, there is an article in there on connecting two phones together over twisted pair. I needs a 9v battery in each unit and a small bit of Veroboard with a 741 to give the gain. It doesn't ring the bell, the 741 is made into an oscillator to make a loud noise in the other end earpiece.
If you can't find the booklet I'll scan it and send it to you - pm me with your email address.