Hello,
To call from the UK to US telephones you would dial: 00 1 followed by the 3 digit area code and 7 digit local number. Unlike the UK, cell phones in the US and Canada use area codes just like landlines do.
If you are in the US with a UK phone dial 011 44 followed by the UK number (you must leave out the starting zero, so if you have a UK cell of the format 07??? ??? ??? you would dial 011 44 7???……).
A third situation: people in the UK dialing a British cell located in the US do not need to add any extra codes to the number they usually dial when the phone is in the UK