Protocols and standards
E.164
Acronym: E.164
E.164 is the standard defining the international format of telephone numbers: a country code followed by the national number, fifteen digits at most.
It explains why a number is written differently depending on whether it is dialled from inside or outside the country. The trunk prefix — the French zero — belongs to the national plan and disappears when dialling internationally, except in countries that use none and where the number is dialled as it stands. Confusing the two is the leading cause of routing failure.