You need to add a SRV record on your external DNS that points to your gatekeeper's IP. SRV records are a new record type that points clients to specific hosts that are running a service (such as H.323)
From: Kirkland, WA
If you already have a published DNS name for a web server or another service, you can use that one. For example we publsish a web server www.ourdomain.com, and for incoming netmmeting calls ourdomain.com is used for the gateway.
Alternatively, you can use the external IP address on your ISA server if you have no public DNS names.