What is the International Numbering Plan for public telephone systems?

The correct answer is the International Numbering Plan for public telephone systems, known as E.164. This plan provides a standardized format for telephone numbers and is essential for ensuring that calls can be successfully routed globally. E.164 defines how phone numbers are structured, detailing that they can have a maximum of 15 digits and must include a country code, which allows for the consistent international dialing of phone numbers. This uniformity is crucial for facilitating communication across different telecommunications providers and national borders.

The other options refer to different aspects of numbering or formatting systems that are not specifically related to public telephone systems. For instance, E.123 pertains to the notation for international telephone numbers in written form, while E.212 outlines the numbering plan for mobile network operators. E.161 is concerned with the mapping of alphanumeric keys on telephone keypads to the digits they represent. While these standards serve their purposes within telecommunications, E.164 is definitive for the structure and functionality of public telephone numbers globally.