I'm not sure what may be causing your problem, but I've used voice chat with no problem with winXP pro, however I don't have a router. If you've forwarded 3783 then it should work, I would do some research on Direct Voice as this is what the game uses. There may be more to it than the Readme indicates. For instance the readme doesn't indicate at all if this is a TCP port or a UDP port, two completely separate animals.
Merry Christmas
Jon