Stu is right, you should go for the skype option, looks this:
3 Store - Price Plans
you buy mobile with data from 3 and skype call are free; buy skypein number (from the USA) and now you get all your calls for very good price.
(you need to check 3 allow skypein on thier mobile; i think yes)
i am sure you can find sheaper offers but go for quality.