i live in the US, and would like to purchase the cell phone game advertised on www.wormsopenwarfare.com, however, my provider is Cingular, and I'm not sure which provider out of the ones shown to choose.

Those look like UK providers to me - the O2 one might work (whenever I've gone to the USA, my O2 phone connects to a Cingular network), but then again it might not...