Windows Live Messenger 8.1 behind SBS2003/ISA Server 2004

I'm trying to get audio/video working with the current version of
Messenger (build 8.1.0178.00) from a Win XP Pro SP2 etc client on an SBS
2003 internal Lan to connect via the internet to a remote PC. Text
conversations work fine.

The SBS box has ISA 2004 installed and connects to the Internet through
a Netgear DSL modem/router (2 Nics installed - internal Lan and external
Netgear router).

If I connect the PC directly to the router and bypass the SBS box,
audio/video seems to work fine. But if I am connected to the internal
Lan, the audio or video call will not connect (either incoming or outgoing).

Messenger's conversation window just says "Connecting with user" and
the video display says "Video call loading" and "Waiting for contact".

Can anyone help in advising how SBS2003/ISA 2004 need to be set up to
allow Messenger audio/video connections to work?

Thanks in advance for any assistance.

Tom Walker