Microsoft MFA (How To) Change a phone number registered for MFA
When people change their phone number or device, they will need to update their Azure MFA with the new number.
To change the phone number assigned in your account for multi-factor authentication:
Log in to Microsoft 365 and click on your Person Icon in the top right corner of your working screen
Choose View account from the drop-down menu.
Click Change next to the method you would like to update.
Edit the phone number and click Next
You will be asked to choose whether Microsoft should call or text the number with authentication codes. Choose the method you prefer.
The following screen will state that Microsoft used the method you chose to send a 6 digit code to your phone. Check your phone for either the call or the text message and enter the code that was sent and choose Next.
Once you have entered the 6 digit code, your new phone number will have been successfully verified and you can click "Done" and close the screen.