I'm not sure it supports UPnP. Unless you have one with new firmware which does support it. Telnet into your device & check the options. If there is none, then port forwarding is the go. See Port Forwarding for the BT Voyager 2091. Most important step is the 1st which is setting up a static ip. The port LW normally uses is port 6346. However you can choose a much higher port such as between 49152 - 65535 such as 64049. But whichever port you choose, you must add the same port number in LW's advanced firewall option as described on that link.

If you're using NIS, make sure XP firewall is disabled. NIS & XP FW's are known to clash & cause problems. Also results in less security with both running.