When Office 365 admin attempts to use office phone extension (x###) for Microsoft Office 365 MFA, you may receive this message: Invalid phone number format. This video shows how to setup authentication method using office phone extension.
The user should do it (not admin)
1. Click on this link:
To access Office 365 Additional security verification page
2. If the Sign in page popup, login using your Office 365 credentials.
- Select your email address.
- Enter your password and click Sign in.
3. In Additional security verification page, you will have these options: Authentication phone (normally it is your smart phone) , Office phone (you can have extension here), Alternative authentication phone and Authentication app or Token. Choose how you want to do your verification.
4. Click Save to save the settings.
Note: 1.Although all options are listed, your admin may not make them all available; you’ll get a message if you choose an option your admin didn’t enable. 2.You also have an option to restore multi-factor authentication on previously trust devices.
Please view this step by step video:
