While few Phone numbers flow as well as Tommy Tutone’s “867-5309/Jenny” song, there is a science behind the way we structure our telephone numbers. Learn why we use 7-digit phone numbers, how area codes became a (necessary) thing and what we used for emergencies before 911 existed.

How phone numbers were invented

As more and more folks bought a telephone after Alexander Graham Bell invented the telephone in 1876, Bell realized he had to develop a better way to get connected than by asking for someone by name.

Back in the early days of phone service, you’d have to pick up the receiver, ask the station operator to patch you through to another person, and then wait for your line to be literally plugged into another person’s line.

The phone number soon started as a 4-digit number sequences instead of names. A caller would share the number with the switchboard operator who could switch the telephone line to the corresponding line number.

Why phone numbers have 7 digits

As the numeric telephone system grew, the initial 4-digit number sequence ran out of unique number combinations. Larger cities began using 5-digit numbers to make more telephone numbers available to people.

Beginning in the late 1920s, alphanumeric codes were assigned to the beginning of numbers to help identify geographic locations. Telephone engineers then devised a 7-digit standard in 1931 to expand their pool of possible number combinations. They also used alphanumeric codes as the prefix to individual line numbers.

The alphanumeric codes were based on the local telephone exchange center’s name. The first 2 letters of the city or  town’s exchange center name would be translated into the first digits of a telephone number, and the last 5 digits would remain the personal phone line code.

For instance, a phone number may be given as MElrose 4-2829. The first two letters of the exchange center name were capitalized to show which letters to translate to numbers. That meant someone would end up dialing 62, the numbers to match ME, before the personal line number 4-2829.

However, the implementation of the 2 letter, 5 number, standard to create 7-digit phone numbers across the U.S. was slow.

The slow move toward 7 digits

Through the 1940s, cities used a variety of telephone number formats such as:

• 3 letters for the exchange center code and a 4 number code for the specific line
• 2 letters for the exchange center code and a 4 number code for the specific line
• 2 letters for the exchange center code and a 5 number code for the specific line

For instance, Lucy and Ricky Ricardo from I Love Lucy gave out their phone number as “Murray Hill 5-9975.” That translated into MUrray Hill 5-9975 or 685-9975.  A Columbus, Ohio, phone book from 1935 instructs readers to dial local numbers using the exchange center name “ADams” and then a four digit number. For instance, a Columbus cleaning service had the number ADams 6226 or 23-6226 in 1935.

When did all phone numbers change to 7 digits?

Some cities kept six figure calling codes into the 1940s, but cities across the U.S. transitioned to 7-digit phone numbers during the 1950s. During this time, all phone numbers in the U.S. transitioned to the 7-digit format of a 2 letter exchange center code and 5 number line code.

The 7-digit format not only standardized exchange center codes and personal phone line codes, it also made phone numbers more distinguishable from one another and made phone numbers easier to remember. And there’s science behind why.

The magical number 7 (plus or minus 2)

Our short-term memory is a finite resource. Countless psychological experiments have shown that, on average, many of us are limited in terms of the amount of information he or she we can receive, process and remember.

In fact, the longest sequence a normal person can recall on the fly contains about seven items. This limit, which psychologist George Miller dubbed as the “magical number seven” when he discovered it in 1956, explains some of the bounds on our capacity for processing information.

Essentially, the brain’s short-term memory can hold 7 chunks of information plus or minus 2 for the sake of variation in human processing speed and capacity. Later on, psychologists also learned that by the use of various techniques like repeated rehearsal, chunking bits of information and using mnemonic devices, humans can slightly expand this informational bottleneck.

And so, the pattern of using 2-digit and 5-digit sequences combined with rhyming patterns of certain number sequences made for more memorable phone numbers.

The birth of area codes, or why phone numbers are 10 digits today

Eventually, 7 digits also weren’t enough to handle the demand of callers hungry for the technology that the telephone dished was dishing out.

Telephone engineers added 3 more digits to the front of the series of numbers to create the area code. Changing phone numbers into 10-digit codes was the next evolution of the original North American Numbering Plan. The update created an efficient, long-distance calling system. Adding an area code prefix would allow people to call others across the country without asking the phone switchboard operator for that person’s phone number.

The prefix area code system and the 3-3-4 dialing scheme developed by AT&T in the 1940s went into effect in 1947 and was first used by telephone operators. It became established as the residential dialing standard in 1951, which helped cement the transition from 6-digit phone numbers to 7-digit phone numbers. The first customer-dialed long-distance call was made that year by Mayor M. Leslie Downing in Englewood, New Jersey, and U.S. residents started with 90 phone area codes in 1951.

How telephone area codes work

Back then, codes were dished out based on population, and the areas with the largest populations (ranging from a whole state to a single city) received codes that were the quickest to dial on a rotary phone.

States that required more than about 500 central phone operator offices, a technical limitation of the number plan, were split into multiple areas. Each successive area in the state received its own code with the middle digit being 1 while area codes for entire states kept the digit 0 in the middle.

The most densely-populated cities had area codes with the shortest pulse sequences. For example, while the first area code was 201 for New Jersey and the District of Columbia received the second code of 202, New York City claimed area code 212, having only five pulses, the shortest of all area codes.

How telephone area codes evolved

With the rapid growth of telephony in the late 20th century, large metropolitan areas also saw the introduction of overlay codes in the mid and late 1990s.

Overlay codes are area codes that cover the same geographical area as another area code. New telephone customers are assigned the new overlay code while existing customers keep using their current area code.

For a time, people could dial others in the same area code as them without adding an area code to the phone number—they’d only have to use 7 digits. Having multiple area codes in the same geographic region meant that all calls in that area needed to include all 10 digits of the phone number.

Overlay codes allow for a larger number of phone numbers to exist within the same geographical vicinity. It also saves telephone companies from having to restructure existing area codes and make customers change their phone numbers.  While residents across the country found the introduction of overlay codes frustrating in the early 2000s, introducing new area codes has become normal.

Before 9-1-1 was THE emergency number

Of course, there are still some phone numbers that don’t abide by the 7-digit and 10-digit rule. And while 911 was eventually adopted as the standard emergency number in the United States and Canada, this was not official until the late 1960s.

Prior to the 911 emergency system, you’d have to call the operator to summon the correct emergency service. In some states, you’d dial “3-4-7-3,” which spells “FIRE,” to reach your local fire department. (You can imagine how effective that was while counting the number of buildings burning down.) In other areas of the U.S., the fire department and police stations had their own 6- or 7-digit numbers just like everyone else.

Even though the standard emergency number was first known as “nine-eleven,” phone companies changed the stylization of it to “nine-one-one” to avoid confusion with people wasting precious time looking for the elusive “11” button that never existed.

Why “555” phone numbers don’t work

If you ever hear a phone number on TV or in a movie, you may notice it’s a “555” phone number. Hollywood often uses telephone numbers that start with 555 because most of them were not real phone numbers until 2016.

Beginning in the 1960s, 555 phone numbers were set aside for entertainment and advertising because the 555 exchange center code wasn’t very popular. Making 555 telephone codes unavailable to the public wouldn’t eliminate a large number of existing phone numbers.

Why 555 out of all the exchange numbers? Well, 5 corresponds to letters J, K and L on the alphanumeric phone dial. Not very many exchange centers started with those letters, so the 555 code wasn’t used much.

As of 2016, The North American Numbering Plan Administration, which assigns phone numbers, has released all 555 numbers outside of the 555-01XX phone number group to the general phone number inventory.  

Not all movies and studios use 555 numbers though. Different production studios buy fake phone numbers to create a more realistic viewing experience. For instance, Universal Pictures owns the number (212) 664-7665 and has used it in a number of its movies.  Call the phone number and you’ll hear nothing but endless rings and rings and rings.

So, the next time you pick up your phone to connect with your friends and family both near and far, remember how much this technology has progressed. You can feel glad knowing that these simple numbers can carry your voice such great lengths.

Shop home phone

Originally published 5/6/16. Updated 8/9/18.

The post Why phone numbers are 7 digits and other fun phone facts appeared first on Allconnect Connected Home Blog.