I have a Polycom ViewStation 512 that I am running 3 ISDN-BRi's on and it works fine to most external locations, however when I try to make a call to a Polycom VSX7000 that we have installed as a built-in to multiple rooms, that also uses Telco ISDN-BRI, the call stops ringing on the ViewStation, but continues to ring on the built-in codec unit the VSX7000.