Mike,
I do recall in the older Quintum products that there was some incompatibility with VocalTec. It seem that there was some sort of signal that the vocaltec was expecting to get from the Quintum and it did not receive it and the call would fail. This signal is not a required signal under H.323 and Quintum had not implemented it in the older version. I believe the newer DX/AS/AX versions may support this feature.
mike