If you are not going soon you can buy a prepaid sim in the US and have your UK phone number before you leave. There is an outfit in San Diego (www.telestial.com) that sells prepaid sims. About a year or so ago I got one from them (Orange) for $40 (US) and that included about 10-pounds (UK) of preloaded calls. With a UK sim all your incoming calls are free and I know with Orange I only pay about 20p a minute for calls to the US.