PSTN Numbering Plans

One feature that slowly changed over time is the dial plan. The addition of second lines for Internet access, cell phones, and fax machines has created a relative shortage of phone numbers. The next section delves into how the PSTN dial plan is put together and what you can expect over the next few years.

In some places in the United States, it is necessary to dial 1+10 digits for even a local call. This will become more and more prevalent as more devices require telephone numbers. The need to dial 1+10 digits for a local number is normally due to an overlay . An overlay can result in next-door neighbors having different area codes. An overlay is when a region with an existing area code has another area code "overlayed." This offers the existing customers the benefits of not having to switch area codes, but forces everyone in that region to dial 10 digits to call anywhere.

Essentially, two numbering plans are used with the PSTN: the North American Numbering Plan (NANP) and the International Telecommunication Union Telecommunication Standardization Sector (ITU-T; formerly CCITT) International Numbering Plan. They are discussed in the following sections.

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