International Telecommunication Union (ITU) recommendation defines the international public telecommunication numbering plan used in the PSTN. It also defines the format of phone numbers. As per these recommendations, numbers can have a maximum of 15 digits and are usually written with a + prefix. CCITT, the predecessor of the ITU Telecommunication Standardization Sector, developed the first formal list of telephone country codes. It was included in the 1964 CCITT Blue Book. The list was organised by creating nine zones. Zone 1 for US, Canada, Bermuda and other Caribbean nations; Zone 2 for Africa and other Atlantic Islands; Zone 3 for Europe; Zone 4 for Europe; Zone 5 for Mexico, South America, Caribbean, and Central America; Zone 6 for Oceania and South-East Asia; Zone 7 for Russia, former Soviet Union; Zone 8 for East Asia and other special services; Zone 9 for South, Central, West Asia and Middle East.
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