E.164 phone numbers: what to store and what a validator cannot know

What it is

ITU-T Recommendation E.164 (cited) defines the international public telecommunication numbering plan: a country code followed by a national significant number, written in international format with a leading +. The libphonenumber source (cited) notes that the ITU sets the maximum length at 15 digits but that the library accepts longer national numbers because they occur in Germany. The project's list of false assumptions (cited) records further limits: the plus sign is part of the format and cannot always be replaced by 00, because the international call prefix differs between countries; in Italy a leading zero after the country code is part of the number; and the country code of a number says nothing about where its owner lives, which time zone they are in or which language they speak.

Why it matters

Phone numbers are identifiers that people type, copy from screens and read aloud. A store that "normalises" them by stripping zeros, converting to an integer or assuming one country produces numbers that reach the wrong subscriber, and a validator that rejects a legitimate format locks real users out of sign-up or two-factor flows.

How to apply

• Store the E.164 form as a string in one column (for example +12025550123), plus the original input and the region used to interpret a national-format entry; never an integer type, which drops leading zeros that are significant in some countries and cannot hold extensions or other dialable characters.
• Parse with a maintained library and its numbering-plan metadata; numbering plans change, so update it the way you update a time-zone database.
• Distinguish "possible" (length and prefix plausible) from "valid" (matches a known range); use the looser check for storing contact data and the stricter one only where it is needed.
• Prove ownership with a one-time code by SMS or voice before a number is used for security; syntax proves nothing about reachability.
• Display in national or international format depending on the reader, but transmit and compare the E.164 form.

Pitfalls

Restricting validation to the site's own region rejects residents with foreign numbers, which the libphonenumber list says arise for many reasons. Short codes, emergency numbers and premium services are not E.164 numbers and need their own handling. Extensions (ext. 12) do not fit the format and belong in a separate field. Two records with the same number in different display formats are the same number; deduplicate on E.164, not on the text as entered.