Cannot receive video calls via messenger

I have tried to start a video message using live messenger 8 with my son,
however although he can start a call from his end and I can receive that call
we cannot speak to each other and cannot see each other on my or his PC. When
I try and start a video or call from my end he does not receive any
notification that there is an incoming call his end, although I do get
notification my end when he calls but there is no voice or video either end I
have sent a message okay. We are using Logitech Messenger webcams and both
have windows XP home. I am using zonealarm and he is using windows fire wall
his end.

Any ideas from the above???

Neil
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Neil