E.164 — ITU-T E.164 international telephone numbering plan
What is E.164?
E.164 is the ITU-T standard for formatting phone numbers so they're unambiguous worldwide: a leading plus sign, then the country code, then the national subscriber number, with the whole string capped at 15 digits. Virtually every messaging and voice API — including Orbit's — requires phone numbers in E.164 form (e.g. +14155552671) for send requests, webhooks, and stored contact records, since it removes any guesswork about which country a bare local number belongs to.
More detail
The format has three parts: the '+' prefix, a 1-3 digit country calling code (e.g. 1 for the US/Canada, 44 for the UK), and the national significant number, with no spaces, dashes, or leading zeros — the full number never exceeds 15 digits. A validation layer at the API boundary typically normalizes or rejects anything that doesn't match this shape before it reaches a carrier.
MSISDN values — the actual dialable numbers assigned to mobile subscriptions — are written in E.164 form; E.164 is the numbering-plan standard, while MSISDN is the telecom term for a specific dialable number that follows it. Because of this, most SMS, voice, and WhatsApp APIs document their phone-number fields as "E.164 formatted" rather than spelling out MSISDN.
Frequently asked
- What does E.164 format look like?
- A leading '+', then the country calling code, then the national number, with no spaces, dashes, or parentheses — for example +14155552671 for a US number or +442071838750 for a UK number. The entire string, including the country code, never exceeds 15 digits.
- Why do messaging and voice APIs require E.164 formatted numbers?
- A bare local number like 4155552671 is ambiguous without knowing the country it belongs to, and formats vary (dashes, parentheses, leading zeros) between regions. E.164 removes that ambiguity with one unambiguous global format, so carriers and routing systems can reliably determine the destination country and validate the number before sending.
See also
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