How to have MSN Messenger voice + video

[Belkin F5D5230 user]

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 ;-)