Transfer of files, voice only worked ONCE....

I have the UPnP (Universal Plug and Play ?) set on my wireless linksys. In
theory, that should be enough for file transfers, voice etc.
I manage to get files transferred, have a voice conversation, even the
remote assistance, all back and forth between an other Messenger user.--ONCE
This was done on a portable computer.. I also put the FTP available on the
Internet connnection in Windows XP.
But the next time I tried with the same user, I could not even send files,
neither could I have a voice conversation.

Could this have anything to do with the fact that the IP is obtained
automatically ?
I really have no clue.
Anybody experienced the same thing.
Thank you
Marc