# E.164 numbers and direct inward dial

Study notes. Not vendor documentation and not legal advice.

- Wikipedia says E.164 is the ITU-T recommendation titled The international public telecommunication numbering plan.
- It says plan-conforming numbers use only digits and a maximum of fifteen digits, with a country code of one to three digits.
- The ITU page that opened lists E.164 (02/26), The international public telecommunication numbering plan, as an in-force component.
- Direct inward dial, on its article, is a service for PBX subscribers: many numbers over trunks, and the dialed number is transmitted so an extension can be reached directly.
- The same article says the dialed number is signaled with DNIS, often in a partial format such as the last four digits.

Write + and then only digits, and stop at fifteen if you are checking the E.164 maximum. Then write the extension and the DNIS digits the PBX actually receives. If a portal shows a different maximum, cite the portal. This room cites the article and the ITU page.