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.
Today, country codes are defined by the ITU-T section of the ITU in standards E.123 and E.164.
In the 1968 White Book, the definition of country codes was relegated to ITU Recommendation E.161.
Codes 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 unlisted whilst being allocated the code "886".