It’s not that simple I’m afraid. H.323 comprises a number of protocols including H.225, Q.931, H.245, RTP and RTCP. The setup procedure is very complicated and the ports are allocated dynamically during call setup. TCP is used for call set up and UDP is used for media transmission.
What you probably need is a Gatekeeper. This will manage admission control to the Internet and, optionally, will route the calls to their destination if they are unreachable from within your LAN.
To have a good look at what’s going on, download Voice2Sniff. It’s a Ethernet sniffer modified for H.323 decoding, and it’s free.