Go to the BT Voyager 2000 program and open 'BT Voyager 2000 User Guide'. Click on 'Wireless Security' and read the section labelled 'Passcode and keys', then read how "To view the WEP Keys generated by the Passcode on your Voyager 2000 base station so devices outside the BT Voyager product range can join your network".
Alternatively just turn off the WEP encryption and use MAC association - less secure but if you limit access to just the host PC and your laptop, it'll stop anyone else accessing your Voyager 2000.