The BT Voyager 2110 is a discontinued product so BT may send you an upgraded router if you call them.
That being said I seem to remember these modems came preconfigured with WEP security enabled. WEP is not secure and Snow Leopard does not play nicely with certain WEP networks. Therefore I would suggest you connect your mac to your BT box via ethernet and goto the routers WebUI
http://192.168.1 and change the Wireless security to WPA2-PSK.
Then your mac should have no trouble connecting wirelessly and you have the added bonus of a much more secure network.