You import a list of contacts into your CRM, and the tool spits back a column of errors. Rejected numbers, duplicates you thought were unique, a "06 12 34 56 78" and a "+33 6 12 34 56 78" counted as two different people when they're the same prospect. The culprit is almost never your software. It's the absence of a common notation rule for phone numbers.
That rule exists, it's international, and it has a somewhat dry name: E.164. Behind that code lies a simple idea — write every phone number on the planet in one and only one way, with no spaces, no dots, no dashes, and the country code in front. Here's the concrete definition of the E.164 format, how to convert a French number in seconds, and why this standardization spares you half your import headaches.
What is the E.164 phone number format?
The E.164 format is the international standard that defines how to write a phone number unambiguously: a + sign, followed by the country code, followed by the national number — all digits run together, with no separator of any kind. It is a recommendation of the International Telecommunication Union, titled "The international public telecommunication numbering plan" (ITU-T, Recommendation E.164).
In practice, the French mobile number you typically write as "06 12 34 56 78" becomes "+33612345678" in E.164 format. The leading zero disappears, the country code +33 takes its place, and all spaces are dropped. One single notation for one single subscriber, anywhere in the world. That is exactly what a database needs: a unique, clean key.
The structure of an E.164 number, digit by digit
An E.164 number reads left to right in three parts: the country code, the national destination code, and the subscriber number. The standard sets a maximum length of 15 digits — country code included — and allows only digits: no letters, no symbols, except for the leading + (ITU-T, Recommendation E.164).
Country codes are one to three digits long: 1 for North America, 33 for France, 49 for Germany, 225 for Ivory Coast. What follows is the national number minus its leading zero. Take the French number "+33612345678": "33" is the country code, "6" indicates a mobile line, and "12345678" identifies the subscriber. Eleven digits in total — well below the fifteen-digit ceiling set by the standard.
Why the + sign changes everything
The + sign at the start of an E.164 number is not decorative: it means "replace me with the international dialing prefix of the country you are calling from." That is what makes a number dialable from anywhere without knowing the local exit code (00 in Europe, 011 in North America).
Without it, a number stays ambiguous. "612345678" alone says nothing about whether it is a French mobile, a landline from another country, or a truncated fragment. With +33 in front, there is no doubt: it is a mobile in France, and it will ring from Tokyo just as well as from Bordeaux. For an international contact database, this prefix guarantees that no number will be misread when placing a call or sending an SMS.
Converting a French number to E.164 format
Converting a French number to E.164 format takes three steps: remove the leading zero, add +33 in front, strip all spaces and separators. French numbers are ten digits long and start with a zero (ARCEP, national numbering plan); that zero is a domestic prefix used only within France.
Here is the full end-to-end example. Start with "06 12 34 56 78". Remove the spaces: "0612345678". Drop the leading zero: "612345678". Prepend the country code: "+33612345678". Done. The same logic applies to landlines: "01 42 68 53 00" becomes "+33142685300". French mobile numbers starting with 06 or 07 are reserved for person-to-person communications under French numbering rules (ARCEP), making them prime targets for prospecting — all the more reason to store them cleanly. If you want to distinguish mobiles, landlines, and business numbers before calling, we cover that in our guide on reachability by number type.
Why normalizing to E.164 prevents CRM import errors
Normalizing all your numbers to E.164 eliminates two plagues of a contact database in one go: invisible duplicates and import rejections. As long as "06.12.34.56.78", "0612 345 678", and "+33 6 12 34 56 78" coexist, your CRM treats them as distinct entries — even though they all refer to the same subscriber.
A normalized database is one where every number has a single form and deduplication finally works. Imports stop choking on exotic formats, call and SMS automations receive data they know how to dial, and you stop reaching out to the same prospect three times thinking they are three different people. This cleanliness is part of a broader data quality effort: before enriching or prospecting, a clean base is non-negotiable. We cover that in our guide on cleaning dead contacts from a list and in the one on data enrichment and compliance.
A well-formatted number is not necessarily a reachable one. E.164 guarantees the form, not the assignment: a number can be perfectly written and still lead nowhere. To verify that a line is actually assigned and active, that is a separate step, which we describe in our page on phone number validity checking.
How to normalize an entire database without doing it by hand
Normalizing a handful of contacts is doable manually; normalizing thousands of rows requires a tool. The logic is always the same — clean separators, handle the leading zero, prepend the country code — but applying it line by line across a large list is as tedious as it is unnecessary.
That is where a prospecting tool that exports directly in E.164 format saves you the step entirely. Whether you extract a list from Google Maps as a CSV or enrich your records with additional phone numbers, getting numbers that are already normalized means no cleanup pass before import. That is exactly what outsend in alpha does: collected numbers come out in E.164 format, ready to drop into your CRM without any reformatting. You start from a raw list and end up with immediately usable data — duplicates removed.
Need a broader overview? See the complete prospecting glossary.
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Request free alpha accessFAQ — E.164 format and phone numbers
What is the E.164 phone number format?
The E.164 format is the international standard, defined by the International Telecommunication Union, for writing a phone number unambiguously: a + sign, the country code, then the national number, all digits run together with no separator. For example, the French mobile "06 12 34 56 78" is written "+33612345678". One and only one notation per subscriber, anywhere in the world.
How do you write a French number in E.164 format?
Three steps are enough. Remove spaces, dots, and dashes; drop the leading zero; add +33 in front. "06 12 34 56 78" becomes "+33612345678", and the landline "01 42 68 53 00" becomes "+33142685300". The leading zero of a French number is a domestic prefix used only within France; it disappears in international format.
Why is there a + in front of E.164 numbers?
The + means "replace me with the international dialing prefix of the country you are calling from" (00 in Europe, 011 in North America). It makes the number dialable from any country without knowing the local exit code. Without it, a number remains ambiguous and can be misread when placing a call or sending an SMS.
How many digits can an E.164 number have?
A maximum of fifteen digits, country code included, according to ITU-T Recommendation E.164. The standard allows only digits, except for the leading +. A French mobile in E.164 format is eleven digits long — well below the limit.
Does the E.164 format guarantee a number is reachable?
No. E.164 only guarantees the written form, not the assignment. A number can be perfectly formatted and still lead nowhere — a cancelled line, a number never assigned, a fake number entered by hand. Checking whether a line is actually active is a separate step, distinct from normalization. Normalization makes the database clean; assignment verification makes it useful.
Why does normalizing numbers prevent duplicates in a CRM?
Because as long as "06.12.34.56.78", "0612 345 678", and "+33 6 12 34 56 78" coexist, the CRM counts them as three separate contacts. By bringing them all to a single E.164 form, each subscriber has only one possible notation, and deduplication works. You stop reaching out to the same prospect multiple times, and your imports stop choking on exotic formats.