To protect your data, a phone number can only be added to a single Microsoft account.
So, after signing into a Microsoft app and you see a message asking if you want to Change number or Transfer number, it means this number is already associated with a different account and you'll need to make a choice about how to continue.
You might see this if:
You created a Microsoft account with that number in the past, but you forgot about it
One of your family members is already using this phone number with their own account
This is a new phone number for you. Since mobile carriers can reuse phone numbers after 30-days, it's possible the previous owner still has that number associated with their Microsoft account.
Tip: For privacy reasons, we can only show a partial view of the account name to give you a hint of what it is.
Your options to fix this
To fix this, you have the option to add a different mobile number (Change number option), or you can transfer the phone number to the account you signed in to the app with.
Use the following guidance to help you select the best option for you.
Use the Change option if:
You have a different mobile number you can use with this app. This will allow you to access all the features of the app.
Use the Transfer option if:
You don’t recognize the partial account (perhaps it's an old account you don't use anymore or it could have belonged to someone else).
You recognize the old account and no longer need that number associated with that account.
Option 1: Change number
If you select Change number, you’ll be brought back to the phone number entry screen where you can select a different mobile number to use with this app.
Select Change number.
Enter a new mobile telephone number.
Follow the remaining prompts to add the new mobile number to the account you signed in to the app with.
Option 2: Transfer number
Selecting Transfer number automatically removes the mobile number from the account it's currently associated with and then transfers it to the account you used to sign into the app.
Select Transfer number.
Enter the password for this account and follow any remaining prompts to add it to your current account.
Note: Transferring the number will prevent you from signing into that account with this number. The next time you try to sign in with that account, you'll be asked to provide a new phone number to sign in with.