What is the maximum number of digits allowable in an E.164 telephone number?

E.164 is an international standard for numbering that establishes a format for telephone numbers to ensure they are unique and universally understandable. According to the ITU-T E.164 recommendation, the maximum length of a telephone number in this format is 15 digits. This includes the country code and the national significant number, providing enough space to accommodate a wide range of telephone number formats used around the world.

The limitation to 15 digits is crucial since it allows for a global standard that maintains consistency across different countries while ensuring that there are enough possible combinations for all users. Each country's telephone numbering plan may vary in length, but as per E.164 guidelines, the total number, including the international dialing prefix, should not exceed 15 digits.

The other provided options exceed the E.164 limit, making them invalid in the context of the maximum allowable digits for an E.164 telephone number.