You are lucky that you have a better firewall than most. Most video conference
systems use h323 or sip to set up calls. The nasty part is they send the IP addresses and port numbers INSIDE this protocol. This make normal nat and firewall rules useless. You end up forwarding huge blocks of ports which defeats the purpose
I forget exactly where you turn this on. You pretty much just turn on the sip/h323 support and it will work. Note there are some VoIP devices that do not follow standard exactly so you will have to test. Polycom is pretty good about following standards.