International calling (or dialling) codes are the country-specific numeric codes appended to the beginning of a phone number when calling internationally. A country-specific international call prefix must also be added before the country code (e.g. "0011" when calling from Australia, and "1666" if calling from Hong Kong).
International calling codes are assigned by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T). The ITU is the United Nations specialized agency for information and communication technologies. More Info
More information on country calling codes can be found at Wikipedia.