This will likely come down to having to understand your firewall rules (work with your network engineers, not against them... even if they can't do the same) and understanding the ip-network-region configurations in your system.
Obviously, connection to PROCR or a CLAN (unpreferred) is needed for h323 registration. After that you need to understand where the RTP packets are flowing. Hopefully all your VPN endpoints are 'captured' by an IPNR meant for these people. Once you have them all grouped you can give them all rules such as no direct media, or direct media only to certain offices etc.
Also, troubleshooting the -now-... get a 1 way call up, do a "status station" and look at the media path. Screen shot that then work with your firewall people to establish that the path is valid for RTP packets. A list trace will also provide similiar data, but the status station during an active call will be a little easier to read.