Tough subject, you would need an application (VoIP) specific firewall that would look into the packet and decode the ASN.1 structure to figure out the IP address and ports. Otherwise, just create big holes in your firewall.
I am looking for some information based on VoIP Gateways cards/PBX. The product I own is an Iter-Tel Axxess 128 system w/VoIP gateway card. The problem is trying to route the static IP through the router which is using NAT. I have 1 IP address being used for my internal network, and am trying to use another for the IP Gateway card. I cant seem to forward packets even with a second router added….