'E.164 is an ITU-T recommendation that defines the international public telecommunication numbering plan used in the PSTN and some other data networks. It also defines the format of telephone numbers. E.164 numbers can have a maximum of fifteen digits and are usually written with a + prefix. To actually dial such numbers from a normal fixed line phone, the appropriate international call prefix must be used.
The title of the original and revision 1 was "Numbering Plan for the ISDN Era".
This recommendation provides the number structure and functionality for the three categories of numbers used for international public telecommunication:
For each of the categories, it details the components of the numbering structure and the digit analysis required to successfully route the calls. Annex A provides additional information on the structure and function of E.164 numbers. Annex B provides information on network identification, service parameters, calling/connected line identity, dialing procedures, and addressing for Geographic-based ISDN calls. Specific E.164-based applications which differ in usage are defined in separate recommendations.
The number categories are all based on a fifteen-digit numbering space (excluding prefixes; before 1997, only twelve digits were allowed). They are as follows:
Number structure for geographic area 
|Country Code||National Destination Code (optional)||Subscriber Number|
|National (significant) number|
|cc = 1 to 3 digits||maximum = 15 − cc = 12 to 14 digits|
|International public telecommunication number for geographic areas (maximum 15 digits)|
Number structure for global services 
|Country Code||Global Subscriber Number|
|cc = 3 digits||maximum = 15 − cc = 12 digits|
|International public telecommunication number for global services (maximum 15 digits)|
Number structure for networks 
|Country Code||Identification Code||Subscriber Number|
|cc = 3 digits||x = 1 to 4 digits||maximum = 15 − (cc + x) = 8 to 11 digits|
|International public telecommunication number for networks (maximum 15 digits)|
Number structure for groups of countries 
|Country Code||Group Identification Code||Subscriber Number|
|cc = 3 digits||gic = 1 digit||maximum = 15 − (cc + gic) = 11 digits|
|International public telecommunication number for groups of countries (maximum 15 digits)|
Recommendations contained in E.164 
This recommendation describes the procedures and criteria for the reservation, assignment, and reclamation of E.164 country codes and associated identification code (IC) assignments. The criteria and procedures are provided as a basis for the effective and efficient utilization of the available E.164 numbering resources. Such assignments require a collaborative effort between the ITU-TSB and the appropriate ITU-T study group to endeavor to ensure that the assignments meet the needs of the telecommunication community. The development of these criteria and procedures are in accordance with the principles contained in E.190 and the numbering plan formats detailed in E.164.
This recommendation contains the criteria and procedures for an applicant to be temporarily assigned a three-digit identification code within the shared E.164 country code 991 for the purpose of conducting an international non-commercial trial.
This recommendation describes the principles, criteria, and procedures for the assignment and reclamation of resources within a shared E.164 country code for groups of countries. These shared country codes will coexist with all other E.164-based country codes assigned by the ITU. The resource of the shared country code consists of a country code and a group identification code (CC + GIC) and provides the capability for a group of countries to provide telecommunication services within the group. The TSB is responsible for the assignment of the CC + GIC.
E.163 was the former ITU-T recommendation for describing telephone numbers for the public switched telephone network (PSTN). In the United States, this was formerly referred to as a directory number. E.163 was withdrawn, and some recommendations were incorporated into revision 1 of E.164 in 1997.