BCM/BST Doorphone can't call across network

[berna54]

I have 5 BCMs, each set up with 3 digit DN's. Dialing a "7" prior to DN will get you to any other BCM, (working over fiber/approx 80 miles between furthest sites).
When using BST doorphone, I cannot figure how to get it to dial out to another system, (group or DN, etc...) About a year ago, I tried working with our vendor, (Verizon) who, after calling Nortel were told that it couldn't be done. Well, I am again revisiting this, is there some way to get the BST to talk across the network to another BCM on the system? If so, HOW??! We are currently using a Viking phone on a POTS line, but that is problematic due to copper quality issues in the area of the Doorphone.
I have tried setting up the doorphone to call a local extension on the local BCM, and tried "forwarding" that phone to the other BCM/facility, but call drops off nearly instantly, (I can watch it happen on the BCM monitor!)

I DON'T need the door opening capability, but I DO need to be able to communicate with Patrons with "one touch" call button. THis is in a setup very similar to an ATM, actually an ATM made for motorists to pay tolls... THANKS.

Any and all help is appreciated