MSN Messanger

Sir,

I have MSN and my two other friends too. We three have same version of msn. if weassume like A, B, C our names A can call to B with video chatb also can chat with video with A and C also. Now the problem is A can’t call with C with video. where is the problem. I google about this find that about UpnP problem. If we have UPNP problem even B also can’t communicate with us with video.our laptops are virus free and every thing is ok. Please tell me.