How to have MSN Messenger voice + video

Hi all!

I wonder why I can't have voice + video with a friend of
mine that's using Belkin router F5D5230.

I gess the problem is the router since I can connect to
all other friends this way (what ever their network
configuration).

Here is a schema of how we connect :

My PC -- Zyxell zywall10 FW Router -- ADSL Modem --
INTERNET

Friend's PC -- Belkin F5D5230 FW router -- ADSL Modem --
INTERNET

Thanks ;-)