Whats about phone numbers?

The North American Numbering Plan (NANP) is a telephone numbering plan that encompasses 25 distinct regions in twenty countries primarily in North America, including the Caribbean and the U.S. territories. Not all North American countries participate in the NANP. Each participating country forms a regulatory authority that has plenary control over local numbering resources.

NPA Number format

A valid NANP number format is as follows:



Country code

Note: “1” is also used as prefix, when making trunk calls from NANP number to NANP number. Within the NANP network, the numbers normally start without the initial “+”.

Permissible characters: all digits.


Zone code

Covers Canada, USA, parts of the Caribbean Sea and some of the islands in the Atlantic and Pacific oceans. The Zone code is usually written in parenthesis.

Permissible characters: [2-9] for the first digit, [0-9] for the second and third digits.


Area or exchange code

This number is normally treated as part of the subscriber’s number. The three digit exchange code is assigned for a certain telephone exchange, which serves the subscribers, but which could physically be remote, using re-addressing, or may belong to cellular operator or other services.

Permissible characters: [2-9] for the first digit, [0-9] for the second and third digits.


Subscriber number

Unique 4 digit subscriber number for subscribers within the range of the telephone exchange.

Permissible characters: [0-9] for each of the digits.

Examples of correct and incorrect telephone numbers

Hereunder you can see examples of correct and incorrect telephone numbers. For each incorrect number, you will find the reason, why it is incorrect.

(234) 235 5678

Correct: All the digits conform to NPA standard.

(123) 234 5678

Incorrect, because NPA cannot start with “1”.

Interesting facts

Each three digit zone code can contain up to 7 919 900 unique telephone numbers:

The NXX can comprise of digits 2-9, which means a package of 8 million numbers: (8×100×10000).

However, both of the last two digits in NXX cannot be equal to 1, to avoid conflict with N11 codes (−80 000).

Despite the widespread use of fictitious number starting with 555, currently only 555-0100 to 555-0199 are reserved for use as fictitious numbers, whereas all the other number starting with 555 are for regular use (-100).

A number of other NXX prefixes are usually not used: local zone code or codes, as well as potential future local codes, technical test codes (usually 958 and 959) and special service codes (for example, 950 and 976).