What Do Digits in Phone Numbers Mean?

By: HowStuffWorks.com Contributors  | 
dialing phone
What do the digits in your phone number represent? Getty Images/Image Source

You probably never think about it, but what are the numbers in a phone number? The easiest way to explain it is the numbers are basically an address similar to the IP address of a computer. How many numbers there are in the phone number varies depending on the country, ranging from just five or six digits in small towns to 10 or more in large cities.

In the United States, phone numbers are fixed-length, with a total of 10 digits. That dates back to the late 1940s when AT&T developed the North American Numbering Plan (NANP) to simplify dialing calls, mostly long-distance. The plan went into effect in 1951. NANP numbers include 10 digits: a three-digit area code and a seven-digit local number. The format is usually represented as NXX-NXX-XXXX.

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  • Area code: Regulated by the Federal Communications Commission (FCC), area codes designate specific geographic regions, such as a city or part of a state. There are 86 original area codes that date back to 1947.
  • 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, systems now allow local number portability (LNP). Today the prefix designates a customer's location, down to the city.
  • Line number: This is the number of the phone line that you are calling. These numbers are assigned to the line and not to the phone itself, which is why 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.

Of course, when you're calling another country, you must first dial 011, which is the international access code, and then the country code. From the U.S., calls can be made to Canada and most Caribbean nations without dialing 011 first. In addition to country codes, some countries also have city codes that you dial after the country code but before the local number.

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Understanding Area Codes

Area Code map
There are so many area codes in the U.S., as seen in this map of just the Northeast. Scott Rothstein/Shutterstock

Today the U.S. has more than 325 area codes in use. Most are assigned to a specific geographic area, though some are not. California currently uses 36 area codes, the most of any state.

When the phone format first came into service in the '40s, blocks of 10,000 phone numbers were assigned to one phone carrier in one region. That was typically plenty of numbers for small and large towns. But in the 1990s, when cell phones burst on the scene, there was a huge demand for new phone numbers. The rise of the internet and voice over IP (VOIP), as well as cable companies, increased the need for more numbers and new area codes.

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For decades, when a new area code was added, they were added by splitting the existing area code into more regions. People living in the most populated areas kept the existing area code, while the rest had their phone numbers reassigned to the new area code.

Instead of splitting the existing area code into more regions, overlay area codes increase the amount of phone numbers available to the same geographic as the original. The introduction of overlay required people to start using 10-digit dialing even for local calls.

So in Georgia, for instance, 404 was the original area code from 1947. Since then, the Atlanta area alone has added numerous area codes to accommodate new phone numbers, including 770, 678 and 470. In March 2022, the Georgia Public Service Commission approved the newest overlay area code for metro Atlanta, 943. New phone numbers with the 943 area code won't be assigned until carriers run out of numbers for the existing codes.

Today all new area codes are added as overlay codes. There are no cities in the U.S. left where you just dial a seven-digit telephone number to complete a call. That ended May 14, 2022, after the FCC designated 988 as the three-digit number for the National Suicide Prevention Lifeline (similar to how dialing 911 from anywhere calls emergency services). That meant 82 area codes in 35 states that used 988 in their local exchange had to transition to 10-digit dialing for all calls.

Starting July 16, 2022, if you dial 988, your call will be routed to the National Suicide Prevention Lifeline.

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Originally Published: Jun 15, 2001

Phone Number Digits FAQs

What is the format for U.S. numbers?
Written as (555) 555-1234, each phone number comprises 10 digits.
What do the numbers in a phone number denote?
The first three digits are the area code, which is unique to each region. The next three digits also pertains to region, but narrows it down a bit. The remaining digits make up the line number.
What is a phone number?
Simply put, it is a sequence of digits that allows us to place calls to specific people residing in different regions through a fixed-line telephone subscriber station, which is in turn linked to a phone line or an electric, wireless device (mobile phone).
Are cell phone numbers listed in a directory?
No, because they are private.
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