From: Brisbane, QLD, Australia
Before I started any ISA configuration I made sure that the public IP address for conferencing on the external ISA and the equivalent on the internal ISA Server were the default IP addresses, i.e. first bound, on the respective interfaces.
For the internal ISA Server I followed Microsoft Article Q303098. I also configured the H.323 Application filter - Select Use This Gatekeeper and enter the internal IP address of the ISA Server.
On the external ISA Server I again used MS Article Q303098 up until the Configuring the H.323 Gatekeeper section. Here, instead of Registering a Static User I added a Destination...
Destination Type - Gatekeeper Destination Address - External IP Address of the Internal ISA Server
Again I configured the H.323 Application filter - Select Use This Gatekeeper and enter the internal IP address of the ISA Server.
This all seems to work fine for a data conference. We haven't had chance to test video and audio yet, but I'm hoping for the best.
Hope this is all clear. If you want more detail I've written up a full step-by-step that I can e-mail you, it just contains private information at the moment.