The international dial code to call the United States of America is +1, according to BT.com. The 1 must be preceded by the dial-out code of the caller's country. For example, the code to call the United States of America from the United Kingdom would be 00 1.
The international dial code from within the United States of America to call out to other countries is 011, as stated by BT.com. As the code to dial the United Kingdom is +44, the code to make a call from the United States of America to the United Kingdom is 011 44.