video conference behind isa server

[alexhendrix 0]

This is the picture, small business server 2003, running ISA server 2000, Exchange 2003, SQL, workstation running winxp pro, the workstation tries to start a video conference using msn messenger, but somehow the signal is dropped, Microsoft says ISA server doesn't allow video conference generated within the network behind the firewall, does anyone knows if there's a way to make it? I've tried starting the video from the outside and it worked in one place but didn't at the other office, same configuration....thanks...