To faciltiate the millions of phone users, the United States of America is divided into regions known as Area Codes, corresponding to particular geographic telephone dialing areas. Based around major cities, some low population states such as Idaho and Montana have only one areas code, whereas other, such as California or New York, may have muliple area codes as they have a requirment for millions of local landline, cell phone, fax, and business phone numbers.
On January 1, 1947, the first North American area codes were issued in regions where standard 7 digit dialing codes were scheduled to be exhaused the soonest. By adding a 3 digit prefix to a 7 digit local number, this allowed for hundreds of millons of new phone numbers to be issued. The North American Numbering Plan Administration (NANPA) is reponsible for further divisions or adjustments to current area codes.