I have under my back not less than 400 installations, configurations and troubleshooted Quintum boxes.
Never met such a problem 🙂
Quintum works perfectly with Cisco, GnuGK, MERA and other softswitches.
With Vocaltec… Well, all kind of equipment has problems with Vocaltec. And the reason of course is in Vocaltec, nowhere else 🙂
issue with vocaltec may raise if you try registering their gateways to another gatekeeper.
A suggestions: change your partner 🙂