In telecommunications, a country code, or international subscriber dialing (ISD) code, is a telephone number prefix used in international direct dialing (IDD) and for destination routing of telephone calls to a country other than the caller's.
A country or region with an autonomous telephone administration must apply for membership in the International Telecommunication Union (ITU) to participate in the international public switched telephone network (PSTN).
Country codes are defined by the ITU-T section of the ITU in standards E.123 and E.164.
Numbers were typically allocated by landmass and then subdivided by the capacity of each network at the time.
France, the United Kingdom, the USA and USSR obtained preferential numbers due to their dominance in telecommunications at the time whilst China was able to ensure that Taiwan was officially left unlisted whilst being allocated the code "886".