The country dialing code for the United States is +1. The United States shares this code with 24 other countries and territories as part of the North American Numbering Plan, or NANP.
The North American Numbering Plan serves 25 countries and territories in total, encompassing a large portion of continental North America, the Caribbean islands, and some U.S. Territories. Not all North America nations participate in the NANP.
The plan was developed by AT&T in 1947 and began implementation in 1951. Due to the NANP, all participating countries and territories share the same numbering resources. As a result, the International Telecommunications Union assigned the +1 calling code to the entire region. The area codes used within the NANP region also use a different format than international calling codes.