How to wire telephones together as a intercom/party-line
A good starting point would be Google :)

It's not overly difficult to do assuming you don't need the phones to ring.  You just need a power source which depends on the length of the cable runs and number of phones.  You can build a small system with a 9V battery even.

If you need the phones to ring it's a bit more difficult as you need to apply 90V A/C at 20hz (I think).

Seriously though, Google it, you will find many walkthroughs and instructions on how to build these.