To place a phone call from the United States to the United Kingdom, dial 011 followed by 44 and the local number. The prefix "011" is the U.S. exit code for placing any international call, while the "44" is the country code of the U.K.
International dialing codes are prefixes to phone numbers that denote call routing to the networks of individual countries. These codes are standardized by the International Telecommunication Union, which was founded in 1865 and is currently an agency of the United Nations. Any country’s code is identical to telephone networks across the world, despite regional differences in the length and format of phone numbers.