Strange problem ???

Anyone who can help me to solve this problem ?

We can not establish the video confernece in Windows XP Messenger vice
versa.
When i invite my friend to start video conference, we can establish
without any problem, but when he is the one invite me to start, we
never establish the video confernece sucessfully.

We disable firewall on both side and still cannot establish the Video
conference. Any suggestion for that ?

I have a Linksys router connect to my compuer with UPnP enable. At the
other side, no router available for him.

Thanks in advance !