E.164 is an international phone number format standard defined by the ITU (International Telecommunication Union). It is the standard format for uniquely identifying phone numbers worldwide, structured as + (plus sign) + country code + national number, expressed in a maximum of 15 digits.
In E.164 format, the trunk prefix (0 in Japan) at the beginning of the national number is omitted. For example, Tokyo's 03-1234-5678 becomes +81312345678, and mobile 090-1234-5678 becomes +819012345678. Hyphens and spaces are not included - it is treated as a pure numeric string.
This standard is important because it eliminates number ambiguity in international communication systems. "03-1234-5678" alone doesn't indicate whether it's a Japanese number, but E.164 format uniquely identifies it from the country code. Therefore, E.164 is adopted as the standard in all scenarios where phone numbers are handled programmatically: VoIP services, SMS sending APIs, CRM systems, and two-factor authentication one-time password delivery.
A practical note: errors in handling the trunk prefix (national prefix) during E.164 conversion are common. Japan's 0 is omitted, but Italy's 0 must be retained as part of the national number - rules differ by country. Also, numbers exceeding the 15-digit limit are invalid in E.164. Google's libphonenumber library can automate accurate E.164 conversion following each country's rules. Review conversion methods in the Japan country code guide.