What is E.164?
E.164 is the ITU standard that defines international phone number format: a leading plus sign, a country calling code, and the national number, up to fifteen digits in total.
A number in E.164 form carries everything needed to dial it from anywhere, with no local context. +15513054842 is unambiguous; 551-305-4842 is only meaningful if you already know it is American.
The rules are strict. Maximum fifteen digits after the plus, no spaces, brackets or dashes when stored, and no national trunk prefix — the 0 that UK and German numbers carry domestically is dropped in the international form.
That trunk prefix is where most integrations break. A UK number stored as 020 7946 0958 becomes +44 20 7946 0958, not +44 020 7946 0958. Our free formatter strips the prefix and validates the length.
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