What kind of distance are we talking about?
Viking makes a couple of units, the DLE-200B
and the DLE-300.
Viking's fax back # is: 715-386-4345
The 200 B is fax back #605 & the 300 is 607
The fax back service is available 24Hrs.&7day
I'm sure these are available through Sundance
and the guys can probably help you as much as the Mfg. You should be able to connect these between CO inputs on each switch.
Program each switch respectivly and when you press that line on on a phone on either switch it should ring the other one.
Hope this Info. helps you out some, Good luck.
WaM