Please do not forget that a good policy established in a firewall works toward the security and benefit of the people behind it, a bad and unmanaged policy just bring a nightmare.
First you have to know that your gateway has several ports to be mapped and the best way to do it, is setting an IP based firewall restriction not a port based one.
This way you can allow Specific IP address use all the ports available on the TCP/IP stack ports like 1720 are for h323 and 1812 and 1813 for accounting and authentications etc.