How a number finds one phone among billions

Fifteen digits at most, country code included. The International Telecommunication Union (ITU) sets that limit in its numbering standard, E.164, the basis for the numbers of every public phone network. A country with a three-digit country code therefore has twelve digits left for its own numbers.

The plus sign and the 00 are not part of the number. They only tell your phone network to send the call abroad, so the standard asks for a plus sign when a number is written down, because the code for abroad differs from country to country.

Sources: ITU-T E.164: the international public telecommunication numbering plan (2026); ITU-T E.123: notation for national and international telephone numbers (2001); Arcep: Décision n° 2018-0881 du 24 juillet 2018, plan national de numérotation (2026)

The ITU does. In the world plan its standards body drew up, each of the digits 1 to 9 opens a world zone: 1 is North America, 2 is mostly Africa, 3 and 4 are Europe. Europe got two first digits because so many of its countries needed a short, two-digit code.

The 1 is shared. The United States, Canada and 18 other countries use the same code and one numbering plan together, so a call from Canada to New York is dialled like a call inside one country.

Sources: ITU-T E.164: numbering plan for the ISDN era (1991); ITU: list of ITU-T E.164 assigned country codes (2016); NANPA: about the North American Numbering Plan (2026)

It depends on the country. Germany has 5,200 local network areas, each with its own area code of two to five digits, and the area code still tells you where the line is.

France dropped the link. Since 1 January 2023, any French landline number can be used anywhere in mainland France. Luxembourg never had it: there are no area codes, and every landline number works anywhere in the country.

Sources: Bundesnetzagentur: Ortsnetzrufnummern (2026); Arcep: La numérotation (2026); ILR: Plan national de numérotation (2026); BIPT: Service description number portability for geographic numbers (1998); Ofcom: The National Telephone Numbering Plan (2025)

Almost every one. A call from abroad first reaches an operator in your country that takes calls from abroad, and it routes the call as it would a call from its own customers. Where that operator asks the database, the call goes straight to the right operator.

The United Kingdom is the exception. There the call still goes to the operator that first held the number, which passes it on with a routing prefix. In 2019 Ofcom, the regulator, wrote that the country was almost, if not entirely, alone in the world in doing it this way, and proposed a shared database.

In Germany the routing number is called a Portierungskennung: the letter D and three digits, put in front of your number. Operators send each other every move, so each one knows where every moved number lives.

Sources: European Conference of Postal and Telecommunications Administrations: ECC Report 31, implementation of mobile number portability in CEPT countries (2005); Ofcom: Promoting trust in telephone numbers, first consultation (2019); Bundesnetzagentur: Portierungskennungen (2026); Bundesnetzagentur: Nummernplan Portierungskennungen, Verfügung 41/2009 (2009); BIPT: Service description number portability for geographic numbers (1998)

Not as the network uses it. A SIM carries its own identity, the IMSI, up to 15 digits that start with a three-digit mobile country code. The ITU standard for it says the IMSI should not be related to your phone number and is not meant to be dialled.

Your operator's register links the two. That is why a moved number can go onto a new SIM from another operator: only the link in the register changes.

Sources: ITU-T E.212: the international identification plan for public networks and subscriptions (2024)

Billions. In 2025 the ITU counted 9.2 billion mobile subscriptions in the world, 112 for every 100 people, and every one of them can be called on a number in this one format.

There are more numbers than phones. By the end of 2016, France's regulator had given operators more than 130 million mobile numbers, while fewer than 85 million SIM cards were in use. Operators need spare blocks to hand out new numbers.

Sources: ITU: Facts and Figures 2025, subscriptions (2025); ITU-T E.164: the international public telecommunication numbering plan (2026); Arcep: Décision n° 2018-0881 du 24 juillet 2018, plan national de numérotation (2026)