Country calling codes or country dial-in codes are telephone number prefixes for reaching telephone subscribers in the networks of the member countries or regions of the International Telecommunication Union (ITU). The codes 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.
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 call prefix must first be dialed. For example, the international call prefix in all countries belonging to the North American Numbering Plan is 011, while it is 00 in most European, Asian and African countries. On GSM (cellular) networks, the prefix may automatically be inserted when the user prefixes a dialed number with the plus sign.