Enter a number to validate its format against the numbering plan for its country, and see how it should be written internationally.
Numbering plans are rulebooks: each country fixes the length of its numbers, which prefixes mean mobile versus landline versus toll-free, and which area codes exist. This validator applies those structural rules for the US/Canada (NANP), UK, Australia, Germany, France and India patterns, plus a general international length check for other countries, and renders the number in national and international (+country) display formats, the merged format-lookup function, since the correct written format follows from the same rules.
The boundary of honesty matters here: format validation proves a number could exist, not that it is assigned, in service, or belongs to anyone in particular. Carrier and owner lookups require paid databases and are not something a web page can do from the number alone, any free tool promising the owner's name is working from leaked or invented data. A format-valid number can still be unassigned; a number that fails these rules is almost certainly mistyped.
The number is valid but calls fail, why? Format validity means the number is shaped correctly for its numbering plan. It may be unassigned, disconnected, or blocked. Validity and reachability are different questions.
Why do test numbers here use 555? In North America, 555-0100 to 555-0199 is reserved for fictional use, so examples in this tool can be perfectly valid in format while ringing nobody, by design.
How do I write a US number for international callers? Drop the leading 1's trunk role into the country code: +1 followed by area code and number, shown in the results as the international format.