Originally designed to identify specific regions like New York (212), Los Angeles (213), or Chicago (312).
First digit 2-9 (avoids special services), second digit 0-9 (cannot match first), third digit 0-9.
With mobile phones and number portability, area codes no longer strictly indicate geographic location.
The next 3 digits identifying the local exchange
The Central Office Code (also called exchange code or NXX) follows the area code and consists of:
Originally identified specific telephone exchanges (physical switching centers) serving particular neighborhoods.
How the first 6 digits fit into the complete US phone number structure
All 10,000 possible numbers in 312305 — fully indexed for search