Phone number formats explained: E.164, country codes and the plus sign

Phone numbers look simple until a form rejects one. The same number can be written with brackets, dashes, spaces, a leading zero, a country code or a plus sign, and different systems want different versions. Understanding the structure takes only a few minutes and fixes many errors, including the common one where a verification code never arrives because the number was entered incorrectly. This guide explains E.164, country calling codes, the difference between national and international formats, and why the plus sign matters when you type a number into a sign-up form.

What E.164 is

E.164 is the international numbering plan defined by the ITU, the United Nations agency for telecommunications. It describes the structure of a phone number so that every number in the world is unique and can be routed. In E.164 a number has a country code followed by a national significant number, and the whole thing is at most fifteen digits. Software writes it with a leading plus sign and no spaces, dashes or brackets, such as +14155550123. Most SMS providers and APIs expect numbers in exactly this form.

Country calling codes

Each country or region has a calling code of one to three digits. The United States and Canada share the code 1, the United Kingdom uses 44, Germany 49, France 33, India 91 and Australia 61. No code is the beginning of another, so a system can read digits from the left and always know where the country code ends. Several countries can share a code: the code 1 covers the North American Numbering Plan, which includes Canada and many Caribbean nations. A code identifies a routing zone, and it does not always correspond to a single country.

National format

National format is how people write a number inside their own country. It often starts with a trunk prefix, a digit dialled before the area or mobile code when calling domestically. In the United Kingdom and many European countries the trunk prefix is 0, while in the North American plan it is 1 for long distance calls. National numbers are also grouped with spaces or dashes in country-specific ways. Because a national format assumes a country, it is ambiguous on its own: the same digits could be valid in several places.

International format

International format removes that ambiguity by including the country code. In writing, it begins with a plus sign. When dialling from a phone, the plus is replaced by the local international access code, such as 00 in much of Europe or 011 in the United States, and many mobile phones accept the plus sign directly. In international format the trunk prefix is normally left out. A UK mobile number written nationally as 07 followed by nine digits becomes plus 44, then 7, then the same nine digits, without the zero.

Why the plus sign matters

The plus sign tells software that what follows starts with a country code. Without it, a string of digits is ambiguous. A sign-up form that receives 4412345678 might read it as a national number in one country, or as a malformed one. With the plus sign, the system can parse it correctly. Many forms add the plus sign for you when you choose a country from a dropdown. If you type it as well, you may duplicate the code. Read the placeholder and any hint text to see which the form expects.

The most common mistakes

Typing the leading zero together with the country code is the most frequent mistake: plus 44 07700 and similar combinations are invalid, because the zero belongs only to the national format. The second is entering the country code in the number field when the dropdown already sets it, which produces the code twice. The third is using spaces, dashes or brackets in a field that accepts only digits. The fourth is selecting the wrong country in the dropdown while typing a number from another. Each leads to a rejected number or a message sent elsewhere.

How to type a number correctly

First, check whether the form has a country selector. If it does, select the correct country and enter only the national number, usually without the leading zero. If there is just a single field, enter the full international number starting with the plus sign and country code, and omit spaces unless the form formats them itself. When copying a number from a website, copy the entire string, including the plus sign. After submitting, read back the number the form shows to confirm that it matches before you request a code.

North American numbers

North American numbers are ten digits: a three-digit area code and a seven-digit local number. In international format they are written with the country code 1, so a number in the area code 415 would be written plus 1, then 415 followed by the seven digits. Within the region, people often write them with brackets and dashes, which forms usually strip. Because the country code is shared with Canada and several Caribbean countries, a form may need you to select the correct country as well as enter the digits.

What developers should do

If you build forms, store numbers in E.164 and show them in a friendly national format. Use a maintained library, such as Google's libphonenumber or one of its ports, to parse, validate and format, instead of writing your own rules, since numbering plans change. Let the user select a country, default to a sensible guess, and accept pasted input in several formats. Store the raw E.164 string, not the formatted one, and validate by sending a code, because a number that looks valid may still not be reachable.

How this relates to public receive-SMS numbers

Every number on this site is shown with its country and in international format so that you can paste it into a form without guesswork. If a code does not arrive, check the format and the country before you blame the number. Public numbers are meant for throwaway sign-ups and testing only, not for accounts that matter. A correctly formatted number does not make a service accept a shared line, but a wrongly formatted one will make even a perfect number appear dead.

Extensions and special numbers

E.164 describes the numbers that can be routed internationally, so it does not cover extensions, which are added after the main number and handled by a company's switchboard. Short codes and alphanumeric sender names are also outside it, since they are local to a country or network. If a service shows a sender such as a brand name or a five-digit number, that is normal, and it is not a malformed phone number.