Simulated telephone

Hello all, I would like to play^H^H^H^H use some of my vintage portables the way they were intended. For example, the Radio Shack Model 100 and the HP 110 Portable have 300bps internal modems. I have several other modems thru which these machines could talk to a *NIX but I do not want to pay for multiple phone lines in my home. How can I simulate a POTS connection between these modems? I don't need a dial tone or ringing, etc., as I can just tell one machine to connect and the other to answer. Way back I heard that a few modems would work if the tip and ring wires of the modems were crossed and connected, but most need something more than that. Is there some simple circuit I could build? Surely I'm not the first to want to avoid phone bills for such play^H^H^H^H testing. Jim