E.164

The international standard format for phone numbers: plus sign, country code, subscriber number, no spaces, max 15 digits.

E.164 is the ITU standard that makes +14155552671 unambiguous worldwide: a leading plus, the country calling code, then the national number, never more than fifteen digits. APIs, databases, and SMS systems normalize numbers to E.164 before doing anything with them.

When a sign-up form rejects a number you know is right, formatting is the first suspect; entering the E.164 form usually settles it. Our free number-lookup tool shows the E.164 form of any number.