Numbering plans are the fundamental for routing the voice calls through PSTN metric. The numbering plans areused to routing different voice calls with different numbering plans. ITU-T E.164 standard is used for creating different numbering plan in USA.North American Numbering Plan (NANP) is also known as “1+10” and is consist of following format.
Where N is representing any number between 2 and 9, whereas X is any number between 0 to 9.
First three numbers NXX are area code, second three numbers NXX are representing prefix for region and final four digits XXXX are called Line number LN.