Understanding US Area Codes & North American Numbering Plan (NANP)

The North American Numbering Plan (NANP) is the unified 10-digit telephone numbering format (NXX-NXX-XXXX) governing telecommunications across the United States, Canada, and 18 Caribbean nations under the administration of NANPA.

What is the North American Numbering Plan (NANP)?

The North American Numbering Plan (NANP) is the telephone numbering system administered by the North American Numbering Plan Administrator (NANPA) for 20 North American countries, including the United States, Canada, and Bermuda. Under NANP, all phone numbers consist of 10 digits structured as NXX-NXX-XXXX.

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Anatomy of a 10-Digit US Phone Number

A standard US phone number comprises three distinct components:

Number Segment	Segment Name	Format Pattern	Permitted Values & Validation Rules
Digits 1–3	Numbering Plan Area (NPA) / Area Code	NXX	First digit N is 2–9; second and third X are 0–9 (e.g. 212, 415, 312)
Digits 4–6	Central Office Exchange Code	NXX	First digit N is 2–9; 555-0100 to 555-0199 reserved for fictional testing
Digits 7–10	Subscriber Line Number	XXXX	Any 4 digits 0000–9999 identifying the individual telephone line
Leading Prefix	Country Calling Code (E.164)	+1	International prefix mandatory for global SMS gateways

(3 1 0)  -  5 5 5  -  0 1 9 9
[Area Code] [Exchange] [Subscriber Line]

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Syntax Rules & Regex Validation for Developers

Under NANP specifications:

• The first digit of the Area Code (N) cannot be 0 or 1 (must be digits 2-9).
• The first digit of the Central Office Exchange Code (N) cannot be 0 or 1 (must be digits 2-9).
• Special reserved exchange prefixes: 555-0100 through 555-0199 are officially reserved by AT&T and NANPA for fictional use in media and software testing. You can generate compliant test numbers with verified area codes and fictional exchanges instantly with our random US phone number generator.

Recommended E.164 Regex Pattern

^\+1[2-9]\d{2}[2-9]\d{6}$

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E.164 International Format Standard

For international SMS integration (e.g., Twilio, Plivo, MessageBird), phone numbers must be formatted in ITU-T Recommendation E.164:

• Leading country code prefix: +1
• 10-digit subscriber number with no spaces, hyphens, or parentheses.
• Example: +13105550199

Generate compliant test phone numbers, standardized addresses, and synthetic developer personas using our Random US Phone Generator, Fake Persona Generator, and US Address Generator.