E.164 is an ITU-T recommendation: the international public telecommunication numbering plan. It is the canonical way to write a telephone number so that any network can route it. The on-the-wire form is a plus sign, a country code, and a national significant number, with no spaces or punctuation required. The telephone-number part (country code plus national number) is at most 15 digits. The plus sign is a prefix, not a digit.
Country codes are 1 to 3 digits, assigned by the ITU. The North American Numbering Plan shares country code 1. A full NANP E.164 number is therefore +1 followed by a 10-digit national number (NPA-NXX-XXXX), 11 digits in total. A German geographic number might be +49 followed by a national number of up to 13 more digits, still within the 15-digit cap.
Structure of an E.164 number
- Plus prefix
- The “+” tells a device or application to treat what follows as an international number. On a PSTN line the caller often dials an international access code (011 in the NANP, 00 in many other countries) instead of “+”.
- Country code (CC)
- 1-3 digits identifying the country or numbering plan (1 = NANP, 44 = United Kingdom, 81 = Japan). Assigned in ITU-T E.164 Annex and operational bulletins.
- National significant number (NSN)
- The remainder, defined by the national numbering plan. It may include a national destination code (area code) and a subscriber number. Trunk prefixes such as a leading 0 are not part of the E.164 NSN.
E.164 vs national vs SIP addressing
| Format | Example (same NANP line) | Where it is used | Pitfall |
|---|---|---|---|
| E.164 | +12025550123 | APIs, SIP URIs, mobile address books, CPaaS | Must not include the national trunk prefix (no +1 0 …) |
| National (NANP 10-digit) | 2025550123 | US/Canada 10-digit dialing, many CNAM and LIDB keys | Ambiguous outside country code 1; do not store this as the only form |
| NANP with trunk prefix | 12025550123 | Some US dial strings and older databases | Looks like E.164 without a plus; parsers disagree on whether the 1 is a country code |
| Local 7-digit | 5550123 | Legacy same-NPA dialing (mostly retired under overlays) | Not unique; unusable for routing or storage |
| SIP URI | sip:[email protected] | SIP signaling for VoIP and SIP trunking | The user part should be E.164. A national-only user part will fail at interconnect. |
Routing, storage, and the 15-digit cap
International outbound calls are routed on the E.164 prefix: country code, then national destination code, then subscriber number. Carriers analyze digits left to right. The 15-digit maximum is a hard ITU-T limit for the international public numbering plan. Private extensions, PBX prefixes, and pause/DTMF suffixes are not part of E.164 and must not be stuffed into the stored number.
A DID is an E.164 number assigned to an endpoint. Local NANP numbers and toll-free 8XX numbers are both E.164 numbers under +1. Formatting for display (parentheses, spaces, dashes) is cosmetic. The digits that matter for routing are the E.164 sequence.