Why Are Telephone Numbers Seven Digits Long?

Seven-digit telephone numbers, the standard format for local telephone numbers in North America, consist of two parts: a three-digit, central office or exchange number (555) together with a four-digit subscriber number (6789).

The First Three Digits

Historically, "central office" was the term used to refer to both the three-digit exchange number and to the building where the local telephone company housed its switching equipment.

Why Three Digits?

In 1919, AT&T Engineers standardized the practice of using three-digit codes. Today, the North America Numbering Plan Administration, the entity designated by the Federal Trade Commission to assign the three-digit numbers, calls these Central Office Codes.

The Last Four Digits

The four-digit number is the actual local "telephone," "subscriber," "line," or "station" number that is assigned to you in the central office equipment used by your telephone service provider.

Why Four Digits?

In the early 20th century, 10,000 to 20,000 lines were the limit for central office switching technology, so four digits (0000 to 9999), which provided blocks of 10,000 numbers, became the standard for each exchange.

"1" For Long Distance

However, seven digits get you only so far. To call your friend two states away, remember that you need 11: first you dial "1," then the area code, then the seven-digit number.

References

• Privateline.com: Telephone History
• NANPA: Numbering Resources

Resources

• The Telephone EXchange Name Project
• The North American Numbering Plan