The telephone number that you dial to call somebody is basically an address, similar to the IP address of a computer or the street address of your home. The length of the telephone number varies depending on the country you are calling. In many European countries, phone numbers are variable in length, ranging from just five or six digits in small towns to ten or more in large cities.

In the United States, phone numbers are fixed-length, with a total of 10 digits. The 3-3-4 scheme, developed by AT&T in 1947, uses three blocks of numbers arranged in two blocks of three and a single block of four digits. Look at the main phone number for as we go through the meaning of the different blocks.

Area code
Line number

  • Area code - Regulated by the Federal Communications Commission (FCC), area codes are used to designate a specific geographic region, such as a city or part of a state. In the example of, the geographic location is the middle section of the state of North Carolina.

  • Prefix - The prefix originally referred to the specific switch that a phone line connected to. Each switch at a phone carrier's central office had a unique three-digit number. With the arrival of computerized switches, many systems now allow local number portability (LNP). This means that a customer's phone number can be moved to another switch without having to change any part of it, including the prefix, as long as the customer does not move out of the local-rate area. For example, moved into new offices in the summer of 2000, but our main phone number did not change. Therefore, our prefix of 882 is not on a specific 882 switch anymore, but our telephone-service provider, Intermedia Communications Incorporated (ICI), is using LNP so that we did not have to change our number.
  • Line number - This is the number assigned at the switch level to the phone line that you are using. Since the number is assigned to the line and not to the phone itself, you can easily change phones or add more phones to the same line.
Think of the three parts like a street address, where the area code is the city, the prefix is the street and the line number is the house. You can even go a step further with this analogy by including the country. The "1" that you dial on long-distance calls within the United States is actually the country code.

Every country has a different country code. To make calls to another country, you must first dial 011, which is the international access code, and then the country code. In addition to country codes, some countries also have city codes that you dial after the country code but before the local number.

There are 680 usable area codes in the United States, of which 215 are currently in use. Each area code has 7,920,000 telephone numbers (out of a possible 10,000,000) available within it. Some numbers, such as those that would start with 0,1 or 911, are unavailable for use. Others, like the area code "555", which is used for fake phone numbers in movies and on TV, are reserved for special use.

The numbers within an area code are split into 792 blocks of 10,000 numbers each. Telephone numbers are given to telephone companies in these large blocks. The biggest problem with this approach is that a lot of the numbers in a particular block may not be used by the company that owns them, but they are unavailable for use by anyone else. This means that the numbers in an area code with several different telephone companies and a growing population can be used up rather quickly.

When a new area code is needed, a decision must be made as to whether the existing area code should be split or the new area code overlaid on the existing one. A split means that the geographic region covered by the existing area code is reduced, and the new area code takes over the remainder. A split requires that a larger percentage of people change their phone number to reflect the new area code. An overlay is an area code that covers the same geographic region as the existing area code. The new overlay area code is given to new customers, and existing customers get to keep their current telephone number. The downside to overlay is that everyone in the geographic region must dial the entire 10-digit phone number, not just the prefix and line number.

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