The international phone code for the United States is 011. The international phone code for a country is also referred to as the country's exit code.
International phone codes differ from country to country. A country's phone code is defined based on the zone in which the country resides. There are nine zones as specified by Telecommunications Standardization Sector, a division of the International Telecommunication Union (ITU). The zones are based on geographical location. The United States and other countries in North America are grouped in Zone One.
For calls within the United States, the code is 1 followed by the city area code.