You can set up your office phone to act as your two-factor verification method. Combined Registration users won't see an option to use an extension with the Office phone option.
Note: If the Office phone option isn't available to select, it's possible that your organization doesn't allow you to use an office phone number for verification. In this case, you'll need to select another method or contact your administrator for more help.
Set up your office phone number as your verification method
On the Additional security verification page, select Office phone from the Step 1: How should we contact you area, select your country or region from the drop-down list, type your office phone number, and then type your extension, if you have one.
In Step 2, you'll receive a phone call from Microsoft, asking you press the pound (#) sign on your office phone to verify your identity.
Copy the app password from the Step 3: Keep using your existing applications area and store it somewhere safe.
Note: For information about how to use the app password with your older apps, see Manage app passwords. You only need to use app passwords if you're continuing to use older apps that don't support two-factor verification.