The North American Numbering Plan (NANP) governs the managment of telephone numbers for the Public Switched Telephone Networks in North America (Canada, USA, Carribean, and territories) within the international country code prefix '+1'.
North American phone numbers are in the format (where N is any digit from 2-9, and X from 0-9):
NPA-NXX-XXXX
Specifically:
NPA identifies the 3-digit Numbering Plan Area (Area Code)
NXX identifies the Central Office (aka. Exchange) within the NPA
XXXX identifies the Station within the NXX
In NPAs that do not require overlay facilities numbers of the form NXX-XXXX are colloquially refered to as local and numbers of the form 1-NPA-NXX-XXXX are known as long distance. In certain NPAs, particularly large urban areas, the maximum number of available NXX and Station code combinations has been reached, requiring additional non-toll NPA's to be assigned - these are known as overlay facilites and their presence is generally reflected by 'local' numbers taking on the form NPA-NXX-XXXX and 'long distance' the form 1-NPA-NXX-XXXX. The NANPA maintains historical assignments statistics on Station and Exchange code use within each NPA and periodically issues exhaustion projections (eg. http://www.nanpa.com/pdf/NRUF/2006-2NPA_Exhaust_Projections.pdf) which are used to advertise and plan network structure changes.
Unlike many countries calls between the majority of local stations in the United States and Canada are toll-free, with only long distance calls incuring a toll charge. The requirement for prefixing a leading 1 to place a toll call is known as requiring toll alerting. The need for toll alerting varies from NPA to NPA however it is universally active in Canada and implemented for most other jurisdictions in the NANP.
The official managment website for the NANP is http://www.nanpa.com
RSI Shadow CMS makes full use of the NANP coordinate system to accurately pinpoint mileage and location names.