E.164 format

The ITU's E.164 recommendation sets a single, unambiguous way of writing numbers from all over the world: a plus sign, the country code, then the national number, all within a limit of fifteen digits.

Concretely, a French 06 12 34 56 78 becomes +33 6 12 34 56 78: the leading zero disappears behind the +33 code. It is this notation that gateways manipulate behind the scenes to route a message without the slightest confusion.