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Glossary · Numbering

What is E.164?

  • Also called: international phone number format, e164 format

Reviewed by Sujan ThapaliyaLast updated

Definition

E.164 is the ITU standard for international phone number format: a leading plus sign, the country calling code, and the national number, with no spaces or punctuation, up to 15 digits total. It is the only unambiguous way to store a phone number.
The three STIR/SHAKEN attestation levels, which underpin how e.164 behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, e.164 included.

In practice

How e.164 actually works

The plus sign is not decoration: it tells the receiving system that what follows is a country code rather than a local number, which is what makes the format unambiguous anywhere in the world.

National trunk prefixes are dropped in E.164. A UK number written 020 7946 0123 domestically becomes +442079460123.

Store E.164 and format for display. Storing the pretty version is how databases end up with the same person recorded four different ways.

Storing numbers in E.164 also removes an entire class of duplicate-record bugs. The same person entered as 020 7946 0958, +44 20 7946 0958 and 442079460958 is three contacts in a CRM and one human being, and every one of those records gets called separately until somebody notices.

+442079460958 — no spaces, no dashes, no leading zero.
PartExampleRule
Plus sign+Replaces whatever exit code applies locally
Country code441 to 3 digits, assigned by the ITU
National number2079460958Trunk prefix removed
Total length15 digits maximumIncluding the country code

Worked example

+442079460123 (UK) · +14155550123 (US) · +919876543210 (India)

E.164: common questions

What is E.164 format?
A plus sign, the country code, and the national number with no spaces, dashes or leading trunk zero — +14155550123. It is unambiguous internationally, which is why every telephony system stores numbers this way.
How many digits can a phone number have?
Fifteen at most, including the country code. National numbering plans set their own lengths within that, which is why a valid number in one country is the wrong length in another.
Do I drop the leading zero when converting to E.164?
Yes. The leading zero is a national trunk prefix used only when dialing domestically. A UK number written 020 7946 0958 becomes +442079460958 — country code, then the number without the zero.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See e.164 in the product

SIP trunking carries your phone calls over the internet to and from your existing PBX, replacing physical ISDN or PRI lines. You keep your phone system and your numbers; you drop the copper, the per-channel rental, and most of the long-distance bill.

Explore sip trunkingGet early access