E.164 Phone Number Format Explained
By SMSOTPS Support Team
E.164 is the ITU international public telecommunication numbering plan. For common API input, an international number is written with a plus sign, country calling code and national significant number. Formatting helps identify the intended number; it does not prove that the number is active or will receive an OTP.
Read the parts of an international number
In the example +14155552671, the plus marks international notation, 1 is the country calling code, and 4155552671 is the remaining national number. The ITU describes geographic-area numbers as a country code plus a national significant number. Twilio's practical guide shows this example and the common plus-and-digits representation.
Keep within the 15-digit maximum
The ITU recommendation limits an international E.164 number to 15 digits, excluding the international prefix. The country code is part of that limit. For a plus-and-digits API field, the plus is not a digit. A string that meets the length limit is not necessarily an assigned, reachable or accepted number.
Do not guess the country or remove zeros blindly
A local number needs country context before it can be interpreted. Follow the relevant national numbering rules when handling a domestic trunk prefix. Do not apply a blanket remove-every-leading-zero rule to all countries. A dialling prefix such as 00 is not the same as a country calling code.
Worked examples of the representation
Twilio's guide shows +14155552671 for the US, +442071838750 for Great Britain, and +551155256325 for Brazil. These are documentation examples, not test destinations. Do not send messages to example numbers.
Follow the form you are actually using
An API may ask for one international field, while an app may provide a separate country selector. Read its input instructions and avoid adding the country code twice. Do not assume every verification form accepts identical punctuation. Copy an assigned number carefully rather than repeatedly changing its digits.
Use formatting tools within their limits
The number formatter uses basic formatting rules, not comprehensive country-aware validation. The dial codes list is a reference. The carrier tool is a demo, not a live carrier lookup. None of these proves ownership, reachability or verification acceptance.
Before you retry a failed verification
Confirm the country, the number as issued, the input field's rules and the service's error message. Correct format does not remove delivery delays or account restrictions. For an SMSOTPS order issue, send support the order ID, without a password or verification code.
Common questions
What is the maximum E.164 number length?
The ITU specifies a maximum of 15 digits, including the country code and excluding the international prefix.
Does correct E.164 formatting guarantee SMS delivery?
No. Formatting does not prove assignment, ownership, reachability or acceptance by the receiving service.
Can the formatter validate every local number?
No. The SMSOTPS formatter uses basic rules. Use country-aware validation and the destination service's input instructions for a production integration.
Related articles
How to Find a Phone Number's Carrier For checking a number: carrier lookup, network information and line type. 
How to Complete SMS Verification Online For a first verification: choosing a service and country, entering the number and reading the code. 
SMS Verification API Guide For developers: API key handling, number orders and reading verification codes.