https://pages.nist.gov/800-63-3/sp800-63b.html
DRAFT NIST Special Publication 800-63B Digital Authentication Guideline Authentication and Lifecycle Management This document and its companion documents, SP 800-63-3, SP 800-63A, and SP 800-63C, provide technical and procedural guidelines to agencies implementing electronic authentication to choose and implement effective authentication processes based on risk. The recommendation covers remote authentication of users (such as employees, contractors, or private individuals) interacting with government IT systems over open networks. It defines technical requirements for each of the three authenticator assurance levels. If the out of band verification is to be made using a SMS message on a public mobile telephone network, the verifier SHALL verify that the pre-registered telephone number being used is actually associated with a mobile network and not with a VoIP (or other software-based) service. It then sends the SMS message to the pre-registered telephone number. Changing the pre-registered telephone number SHALL NOT be possible without two-factor authentication at the time of the change. OOB using SMS is deprecated, and will no longer be allowed in future releases of this guidance.
