The dialing code to make phone calls from the United States to the United Kingdom is 011 (U.S. exit code) and then 44 (U.K. Country Code). To place a call, dial 011, then 44, then the phone number.
A U.K. landline number consists of the area code, which is from two to five digits long, and the phone number, for a total of 10 digits. A sample call from the United States to a London land line is: 011 44 20 xxxx xxxx (where 20 is the area code for London). Cellphone numbers in the United Kingdom also have 10 digits, with seven always being the first one. A sample call from the United States to a U.K. cellphone is: 011 44 7 xxx xxx xxx.