North American Numbering Plan (NANP). Invented in 1947 by AT&T and Bell Laboratories, NANP is the numbering plan for the Public Switched Telephone Network (PSTN) in the United States and its territories, Canada, Bermuda, and many Caribbean nations, including Anguilla, Antigua & Barbuda, Bahamas, Barbados, British Virgin Islands, Cayman Islands, Dominica, Dominican Republic, Grenada, Jamaica, Montserrat, St. Kitts and Nevis, St. Lucia, St. Vincent and the Grenadines, Trinidad and Tobago, and Turks & Caicos. NANP is what keeps any two people/businesses/whatever from having the same telephone number.