E.164 phone number format

E.164 is the international standard for phone numbers, defined by the ITU. It is the format MX8 Labs uses for voice and text respondent sources because it unambiguously identifies a single subscriber line anywhere in the world. Twilio, every major telecoms carrier, and most contact-management systems all use E.164, so contact lists exported from CRMs, billing systems, or panel providers can usually be loaded directly without reformatting.

What an E.164 number looks like

An E.164 number is written as:

a leading + sign
the country calling code (1 to 3 digits)
the subscriber number (national significant number), with no leading 0
no spaces, hyphens, parentheses, or other punctuation
no more than 15 digits in total (excluding the +)

Most US numbers are simple to convert: take the 10-digit number and prepend +1. Parentheses, dashes, dots, and spaces are all stripped.

Local US format	E.164 format
(212) 555-0143	`+12125550143`
415-555-0190	`+14155550190`
312.555.0177	`+13125550177`
305 555 0162	`+13055550162`
1-800-555-0199	`+18005550199`
646.555.0123	`+16465550123`
(702) 555-0188	`+17025550188`
1 (888) 555-0145	`+18885550145`

The same +1 prefix covers Canada and most of the Caribbean, since all of these share the North American Numbering Plan (NANP):

Country / region	Local format	E.164 format
Canada	(416) 555-0199	`+14165550199`
Puerto Rico	(787) 555-0134	`+17875550134`
Dominican Rep.	(809) 555-0156	`+18095550156`
Jamaica	(876) 555-0172	`+18765550172`

For numbers outside North America, drop the leading 0 (the "trunk prefix") that many countries use for national dialing, then prepend + and the country code:

Country	Local format	E.164 format
Mexico	55 1234 5678	`+525512345678`
United Kingdom	07911 123456	`+447911123456`
Germany	030 12345678	`+493012345678`
Australia	0412 345 678	`+61412345678`

So a UK mobile written nationally as 07911 123456 becomes +447911123456, not +4407911123456.

Common country prefixes

The vast majority of US contact lists will use +1. The other prefixes below are the ones most commonly seen alongside US numbers — in mixed-country lists, expat panels, or studies that field across North America, the UK, and Western Europe.

Prefix	Country / region
`+1`	United States, Canada, and most of the Caribbean (NANP)
`+52`	Mexico
`+44`	United Kingdom
`+33`	France
`+49`	Germany
`+34`	Spain
`+39`	Italy
`+31`	Netherlands
`+353`	Ireland
`+61`	Australia
`+64`	New Zealand
`+81`	Japan
`+86`	China
`+91`	India
`+55`	Brazil
`+57`	Colombia
`+54`	Argentina
`+972`	Israel
`+971`	United Arab Emirates

For anything else, the full ITU list is at itu.int.

Common conversion mistakes

Forgetting the +1 on US numbers — 2125550143 and 12125550143 are both invalid; only +12125550143 is E.164.
Including formatting characters like spaces, hyphens, parentheses, or dots — +1 (212) 555-0143 is not valid; +12125550143 is.
Storing the number as an Excel number, which silently strips the leading + (so +12125550143 becomes 12125550143) — always format the column as Text before pasting numbers in.
Using 00 instead of + (0012125550143 is not E.164).
For non-US numbers, leaving in the trunk prefix 0 after the country code (+4407911... instead of +447911...).
Truncated numbers from CSVs that were opened in Excel and re-saved.

Where E.164 is required

E.164 is required for the Identity column in:

Numbers that are not in E.164 will be rejected at upload time. The identity column must also be unique within the contact file — duplicate numbers will cause the upload to be rejected.