What is E.164?
E.164 is the international phone number format: + country code + national number, max 15 digits — +14155552671. It is the only format APIs, HLR lookups, and databases should ever store.
Definition
E.164 is the ITU-T recommendation that defines the international public telecommunication numbering plan — in practice, the canonical format of a phone number: a plus sign, a 1–3 digit country calling code, and the national significant number, totalling at most 15 digits with no spaces, dashes, or brackets. +14155552671 and +442071838750 are E.164; “(415) 555-2671” and “07 183 8750” are local dialling habits that only make sense in one country.
E.164 matters because it is the unambiguous join key of the telecom world. The same national number can exist in dozens of countries; only the E.164 form is globally unique. Every serious API — SMS gateways, HLR lookups, WhatsApp, caller ID databases — expects it, and every deduplication or fraud system that stores numbers in local formats eventually double-counts subscribers. Normalisation is non-trivial: leading zeros drop, national prefixes differ, and formats like Argentina’s mobile 9-prefix have edge cases — which is why dedicated parsing libraries and services exist.
Why it matters
Every phone-number bug you will ever ship traces back to not normalising to E.164 at the boundary: duplicate user accounts, OTPs sent to malformed numbers, lookups that miss. Normalise once, on input, and store nothing else.
See it in action
Normalise any number with the E.164 parser →