I have found RebTel (www.rebtel.com) to be the easiest and cheapest. I assign a UK number to a person I want to call. Then with my unlilmited landline bolt-on my calls to the States are free. (With exception to what I pay for the bolt on, obviously). The person on the other end also gets sent a U.S. number they can call me on.
Do you happen to know which number the other person sees you're calling from? Your mobile number or a different one from RebTel?