Dette står på følgende side
http://support.microsoft.com/default.aspx?scid=kb%3Ben-us%3B278887
Men hvordan gør jeg det?
"To work around this problem, install Proxy server, create a Winsock
protocol definition for TCP ports, and then install the Winsock proxy client
on the computers that use the proxy server.
Winsock Protocol Definitions
To enable voice communications, make sure that outgoing TCP connections from
port 6901 are enabled. Also enable UDP packets where either the source or
the destination port is 6901.
To enable file transfers, enable both incoming and outgoing TCP connections
to the 6891-6900 range of TCP ports. This allows each sender to perform up
to 10 simultaneous file transfers. Note that if only TCP port 6891 is open,
users are only able to perform one file transfer at a time."
S.G, Odense