I have spent a fair amount of time trying to get this to work but it is now staring to drive me mad! I've done absolutely everthing that Tom Shinder said to do in his article, but external clients still cannot call in.
I have configured the H.323 Filter to allow both incoming and outgoing. I have a protocol rule that allows the H.323 protocol for 'Domain Users'. I have a Site and Content Rule that allows any request to any site. The gatekeeper is configured to use the internal interface, and the internal clients are successfully connected to it. The external client is configured to use the external interface of the ISA server as the gateway.
Connecting Internal to Internal, or Internal to External all works fine, but the very same External client simply cannot connect (by phone number) to the internal client!
Important... all the articles here provide a lot of useful information, but not one of addressed the issue I was having.
However, it is very important so... if you have more than 1 IP address assigned to the external interface of your ISA server, external clients will not he able to connect with NetMeeting at all unless you use the default (or first) IP assigned to the interface.