First, you must download Microsoft Authenticator and register your Office 365 account within Microsoft Authenticator:
- Browse to https://aka.ms/mysecurityinfo.
- Sign in, then add the Authenticator app by selecting Add method > Authenticator app, then Add Method.
- Follow the instructions to install and configure the Microsoft Authenticator app on your device.
- Select Done to complete Authenticator configuration.
- In Microsoft Authenticator, choose Enable phone sign-in from the drop-down menu for the account registered.
- Follow the instructions in the app to finish registering the account for passwordless phone sign-in. Note that you may be required to change some settings on your phone for this to work. For example: if you don't have a passcode lock on your phone, you'll be required to enable it.