I have been using ISA, to support netmeeting calls. I have no problems with audio, or video, if the remote client machine, is not behind any type of firewall. I now have a client who is behind a linksys 10 port router. When this cleint uses netmeeting to call my machine behind ISA, i can hear the client, but the client cant hear me, and no video works either. Application sharing works, and file transfer works. If i can hear the client, but the client cant hear me, then which firewall is blocking these packets, the linksys on the remote network, or ISA?
If the firewall or NAT device at the remote site doesn't support explicitely the H.323 protocol, you will definitely have problems with the audio and video part of the conversation. The data part should work if the remote site initiates the connection. So, you have to find out if the Linksys has special support for NetMeeting or H.323.
BTW --- if you are interested to find out why it doesn't work, use the free Network Monitor Ethereal. There is a plugin available for decoding the H.323 protocol used by NetMeeting and the ISA H.323 Gateway/Gatekeeper.