E.164 is an international standard, titled The international public telecommunicationnumbering plan, that defines a numbering plan for the worldwide public switched telephone network 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 field of the signaling messages. When dialing, the number must typically be prefixed with the appropriate international call prefix, 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:
The E.164 recommendation provides the telephone number structure and functionality for three categories of telephone numbers used in international public telecommunication: For each of the categories, it details the components of the numbering structure and the digit analysis required for successful routing of 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. Before 1997, only twelve digits were allowed. The definition does not include any international call prefixes, necessary for a call to reach international circuits from inside the country of call origination.
Geographic areas
Global services
Figure 2
Networks
Groups of countries
Recommendations
E.163
E.163 was the former ITU-T recommendation for describing telephone numbers for the public switched telephone network. 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 E.163 was withdrawn, and some recommendations were incorporated into revision 1 of E.164 in 1997.
E.164.1
This recommendation describes the procedures and criteria for the reservation, assignment, and reclamation of E.164 country codes and associated identification code assignments. The criteria and procedures are provided as a basis for the effective and efficient utilization of the available E.164 numbering resources.
E.164.2
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.
E.164.3
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 and provides the capability for a group of countries to provide telecommunication services within the group. The Secretariat of the ITU Standardization Sector, the Telecommunication Standardization Bureau is responsible for the assignment of the CC + GIC.
DNS mapping of E.164 numbers
Some national telephone administrations and telephone companies have implemented an Internet-based database for their numbering spaces. E.164 numbers may be used in the Domain Name System of the Internet in which the second-level domain e164.arpa has been reserved for telephone number mapping. In the system, any phone number may be mapped into a domain name using a reverse sequence of subdomains for each digit. For example, the telephone number translates to the domain name 3.2.1.0.5.5.5.9.9.9.1.e164.arpa. When a number is mapped, a DNS query may be used to locate the service facilities on the Internet that accept and process telephone calls to the owner of record of the number, using, for example, the Session Initiation Protocol, a call-signaling VoIP protocol whose SIP addresses are similar in format to e-mail addresses. This allows a direct, end-to-end Internet connection without passing to the public switched telephone network and without incurring PSTN tolls. As this is effectively a free call, there is little incentive for carriers to promote e164 DNS service. The e164.arpa domain is in production status only in a few European nations. Australia conducted a trial in 2007, but then abandoned further support of .1.6.e164.arpa. Many nations have no .e164.arpa implementation active.
