Definition
E.164
The international standard format for phone numbers: a plus sign, country code, and up to 15 digits.
E.164 is the canonical way to represent a phone number unambiguously — for example, +14155550123. Normalizing numbers to E.164 is the first step in any validation pipeline, because it removes formatting inconsistencies and lets you reliably parse country and national portions.
Numbers that can't be parsed into valid E.164 are the cheapest to drop from a list before any deeper lookup.