Country calling codes or country dial in codes are telephone dialing prefixes for the member countries of the International Telecommunication Union (ITU). They are defined by the ITU-T in standards E.123 and E.164. The prefixes enable international direct dialing (IDD), and are also referred to as international subscriber dialing (ISD) codes.
Worldwide distribution of country calling codes colored by first digit Country codes are a component of the international telephone numbering plan, and are necessary only when dialing a telephone number to establish a call to another country. Country codes are dialed before the national telephone number.
By convention, international telephone numbers are represented by prefixing the country code with a plus sign (+), which also indicates to the subscriber that the local international dialing prefix must first be dialed. For example, the international dialing prefix in all countries belonging to the North American Numbering Plan is 011, while it is 00 in most European countries. On GSM (cellular) networks, the prefix may automatically be inserted when the user prefixes a dialed number with the plus sign.