Where is the installer on all of this? Surely that company has been involved, what are their recommendations? Whatever you end up doing will require the expertise of an on site technician.

As NFC said, likely the best/least expensive solution will be to use a VoIP card at both locations and route the calls through your VPN. It might cost more up front, but it should give the features you desire. A Multitech VoIP box could help if you want to go cheaper, but it will not give feature transparency like connecting directly with Telrad equipment will.

In summary, you should get a qualified Telrad sales person and tech to explain exactly what the options are and how to implement them.

Steve