Well, there are a few ways to do this. The easiest way would be to use a 401B manual intercom card in your 620 panel to mimic the talk battery of a CO line. You would then connect the tip and ring of each phone to the intercom card's T/R output from the panel. Of course, you wouldn't be dialing anything to connect them; they'd always be connected to each other via this parallel connection.

You could also put a pair of automatic tie line cards in your panel, but those are nearly impossible to find.

Lastly, you could purchase an automatic ringdown device, such as this one from Viking Electronics that would ring the other set whenever one goes off-hook. It is a stand-alone unit that won't need any slots or power from your panel:

VIKING DLE-200B

Ed Vaughn, MBSWWYPBX