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What does NPA/NXX stand for?

“NPA” stands for Number Plan Area, commonly called Area Code. “NXX” refers to the three digits of a phone number immediately following the area code, also called the “exchange” or the “Central Switching Office Designation.

In the number (555) 222-3333, the NPA is “555” and the NXX is “222”.

The “N” numbers, or the first number of the NPA and of the NXX, can be any number from 2-9; 0 and 1 are reserved for special purposes. All the remaining numbers, or “X” numbers, can be any number from 0-9.

The last four digits of a phone number are called the “Subscriber Line Identifier,” or “SLID”, and can be any number from 0-9..

What does NANP stand for?

North American Number Plan. This is the body that manages North American telephone numbers on public switched networks in regions where the international country code is “1”. That includes the US and it's territories, Canada , and the Caribbean .

What is “number depletion”?

Number depletion refers to the shrinking number of phone numbers available for distribution. Some of the causes of number depletion are the increase in demand for new phone numbers for cell phones and pagers.

How many numbers are available in each area code?

Each area code has 7,920,000 different usable phone numbers.

(Although there are technically 10 million seven-digit phone number combinations available in each area code, some numbers aren't available, such as 911, 411, and numbers starting with 0 or 1.)

Why are new area codes given out sometimes?

Due to “number depletion”, some areas could conceivably run out of phone numbers – at that point, the area code would be “exhausted”. Before an area code is exhausted, a plan is put into place to provide “relief” for the region to make way for new phone numbers.

Several different methods have been used to provide this relief.

  • Area Code Realignment : This relief method shifts part of one area code into a neighboring area code – a block of phone numbers change over to the area code of a neighboring region with fewer phone numbers - to free up numbers in their original area code.
  • Area Code Split : In this relief plan, one region is split into two (or more) based on a geographic line, with one portion keeping it's original area code and the other getting a new area code.
  • Area Code Overlay : In this relief scenario, existing phone numbers do not get a new area code, but all new phone numbers do. This means that one geographic area has literally two area codes – your neighbors on either side of you could have a different area code than yours – in fact, two phone numbers in the same building could have different area codes.

A single region may have more than one overlay – that means it can have more than two area codes.

In an overlay region, all local calls require the area code be dialed, even if you are placing a call to the same area code as your own.

What are “Exchange Names”?

Exchange names refer to the first digits of a 7-digit phone number. They were introduced in the 1930's and used until around 1970. The first 2 or 3 numbers (not including the area code) were identified by location names, often the first two letters of the name of the Central Office that served the location.

Here are some famous examples:

BUtterfield 8 : This 1960 movie starring Elizabeth Taylor is named for a telephone exchange, and the “U” in BUtterfield is capitalized to call attention to this. All the characters in the film were in the same telephone exchange, meaning all of their phone numbers began with BU8. Today, we'd simply dial 288, the corresponding numbers.

Pennsylvania 6-5000 : This phone number (736-5000 by today's terminology) was immortalized by Glenn Miller and his orchestra in 1940. It was the phone number of the Hotel Pennsylvania in New York City , where Miller opened at the Café Rough in that hotel. Following that three month engagement, the band recorded the song by Jerry Gray and Carl Sigman with the title and lyrics (“Oh baby just call - Pennsylvania 6-5000”) commemorating the hotel's phone number.

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