What Makes a Phone Number Valid?

A valid number has the right length and prefix for its country's numbering plan. Valid does not mean it is in use or can receive texts.

Three different checks

Why apps say "invalid number"

• A missing or wrong country code.
• A leading 0 after the country code.
• Too many or too few digits.
• The app refuses the number type, such as some calling-app numbers.

Checking a number

Libraries such as Google's libphonenumber check format. Checking type and reachability needs a carrier lookup. See how to check if a number is VoIP.

Need valid-format test data? Use our generator, in fictional mode where possible.

Testing forms safely

When testing your own sign-up form, use fictional numbers from ranges reserved for testing where a country has them. They pass format checks without reaching a real person.

FAQ

What makes a mobile number valid?

The right length and prefix for its country.

Does valid mean the number exists?

No. It may not be assigned or switched on.

Why does an app say my number is invalid?

Usually a country code or leading 0 problem, or a refused number type.

Related: check if a number is VoIP · how to write a phone number · phone number generator.