Which organization is responsible for defining the E.164 numbering plan?

The correct answer is the International Telecommunication Union (ITU), which is responsible for defining the E.164 numbering plan. E.164 is a recommendation by the ITU that outlines how telephone numbers are structured and formatted for use in the public switched telephone network (PSTN). This numbering plan provides a standardized format for ensuring that international calls are efficiently and accurately routed between different countries and telephone systems.

The ITU plays a crucial role as a specialized agency of the United Nations, focusing on international telecommunications and establishing important global standards, including E.164. This ensures global consistency in the way telephone numbers are assigned, allowing for interoperability across different nations' telecom systems.

Other organizations mentioned, like the IETF (Internet Engineering Task Force), specialize in internet standards and protocols, but do not deal with global numbering plans like E.164. The ISO (International Organization for Standardization) focuses on a wide range of standardization issues but does not define telephone numbering. The CCITT (Comité Consultatif International Téléphonique et Télégraphique) is an earlier organization that was absorbed into the ITU and originally contributed to uniform telecommunication standards, but it is no longer a standalone body. Therefore, the ITU remains the definitive authority on E.164