E.164 is an international standard (ITU-T Recommendation), titled The international public telecommunication numbering plan, that defines a numbering plan for the worldwide public switched telephone network (PSTN) and some other data networks.

E.164 defines a general format for international telephone numbers. Plan-conforming numbers are limited to a maximum of 15 digits, excluding the international call prefix. The presentation of a number at the B-party device is usually prefixed with the plus sign (+), indicating that the number includes the country calling code. This is done by the B-party subscribers network by usually looking at the NOA (Nature Of Address) field of the signaling messages. When dialing, the number must typically be prefixed with the appropriate international call prefix (in place of the plus sign), which is a trunk code to reach an international circuit from within the country of call origination.

As described in by the ITU, the E.164 general format must contain only digits split as follows:

Alternative formats (with area codes and country specific numbers) are available.

The title of the original version and first revision of the E.164 standard was Numbering Plan for the ISDN Era