MSN Mess = Video No / Win Mess = Video Yes

I have a Linksys BEFW11S4 ver 4 with the latest firmware and UPnP enabled.
When I attempt a video conference with Windows Messenger 5 to a MSN
Messenger 6 user it works fine. If I try a video conference with MSN
Messenger 6 to a MSN Messenger 6 user, it fails after attempting to
connnect. If I put the computer in the router's DMZ MSN Mess works fine. I
looks like MSN Mess is not opening the proper ports but Windows Mess is.

Does anyone have any thoughts?

Thanks.