[bbob]

There is none as far as I know.

The roaming sims that offer free roaming on their uk number when in the usa are allready a very good deal. Normally in the usa as owner of a cellphone you also pay for receiving incoming calls (unless you have flatrate).

If you do want a usa number you can always use a serivce where the usa number is rerouted to your uk mobile. This will still cost you money but might be a bit cheaper thatn someone calling your usa number.

All really depends on how much you think you are going to use it.