Area codes are also known as Numbering Plan Areas (NPAs). These are necessary (for the most part) only when dialed from outside the code area,from mobile phones,and (especially within North America) from withinoverlay plans. Area codes usually indicate geographical areas within one country that are covered by perhaps hundreds of telephone exchanges,although the correlation to geographical area is becoming obsolete.[ 2 - It must usually be preceded in the dial string by either the national access code or the international access code and country code. For non-geographical numbers,as well as mobile telephones outside of the United States and Canada,the "area code" does not correlate to a particular geographic area.Area codes were introduced in the United States by theBell System,in 1951,with the first direct dial long distance call.[ 3 - Local number The local number (or subscriber number) must always be dialed in its entirety. The first few digits in the local number typically indicate smaller geographical areas or individualtelephone exchanges. In mobile networks they may indicate a network provider in case the area code does not. Callers from a number with a given area/country code usually do not need to (but optionally may) include the particular area/country code in the number dialed,which enables shorter "dial strings" to be used. Devices that dial phone numbers automatically can include the full number with area and access codes,since there is no additional annoyance related to dialing extra digits.