Depends on whether your ADSL router/firewall has dynamic h323 support. If not:-
Find out what UDP ports the gateway is using for voice and configure these for static PAT on the ADSL router. Also configure the standard H323 signalling ports as static PAT on the router. I think it will also help if your h323 gw allows you enetr a NAT ip address in the config. That way ti will use the right IP when it embeds its IP in teh H245 messages…#
Hope this helps…