E.164 is an international standard public telecommunication numbering plan that defines the worldwide public switch telephone network and some data networks. In other words, it defines the general format for international telephone numbers.
Phone numbers that conform to e.164 are limited to 15 digits maximum, excluding country codes. Numbers displayed on recipient devices are usually prefixed with the “+” symbol to indicate that the number includes the country code.
Recipient network subscribers are able to determine this usually by looking at the NOA (nature of address) field of signaling messages.
Generally e.164 number format must only contain digits, spilt by country code (1 to 3 digits), followed by subscriber/recipient number (max 12 digits).