IDDD depends as directly on numbering as does national DDD, but standards are now a matter of global concern. The Comite Consultating International Telegraphique et Telephonique (CCITT) foresaw this need and organized a study to determine how to satisfy it. The standard, approved in 1964, establishes eleven digits as a preferred maximum length for international numbers but allows twelve.

The international number is flagged by a dialed prefix, not internationally standardized, that alerts the switching equipment. The international number itself consists of a country code and a national number. Country codes are standardized and vary in length from one to three digits, the first digit of which constitutes the world zone number. National numbers are the familiar telephone numbers used for domestic long-distance service.

The countries or zones anticipating the greatest telephone population by the year 2000 were assigned the shortest country codes to allow for longer national numbers. Specifically, the unified North American world zone is 1 and the Soviet Union zone is 7. In these cases, world zone and country code are the same. Other zones contain a mix of 2-digit and 3-digit country codes (for example, 52 for Mexico and 502 for Guatemala). Countries where the number of telephones to be served can be handled by nine or fewer digits are assigned 3-digit codes. Certain combinations of the initial two digits (22, 23, 24, 25, and 26 for world zone 2; 35 for world zone 3; 50 and 59 for world zone 5) are selected in forming 3-digit codes. The other pairs are assigned as 2-digit country codes. Thus, from the initial two digits, switching systems can determine whether the country code is two or three digits long. The dialing sequence for an IDDD call is illustrated by the following call from England to the United States. The customer in England dials 010-1-NXX-NXX-XXXX/ where 010 is the international subscriber dialing prefix used in England; 1 identifies North America as the world zone (and/ in this case, is the country code); and the remaining digits are the familiar 10-digit address or national number used in North America. The Bell System has authorized two prefixes for outwardbound IDDD. The prefix Oil indicates simple coin or noncoin automatic calls. The prefix 01 indicates a desire for operator assistance.


World Zone Principal Areas Covered

1 2 3, 4 5 6 7 8 9 0

Canada, United States Africa Europe South and Central America, Mexico South Pacific U.S.S.R North Pacific Far and Middle East Spare