problem changing my phone number

Select Transfer number when prompted, because the “number is already in use” message means the corrected number is associated with another Microsoft account.^1^

Move the correct number

1. Return to the screen where you entered the corrected phone number.
2. If Transfer number appears, select it and complete the verification prompts.
3. If Microsoft identifies the number as an account itself, select Sign up and follow the prompts to remove it from the existing account and move it to your current account.

If the other account belongs to you but no transfer prompt appears:

1. Sign in to the Microsoft account currently holding the correct number.
2. Ensure that account has another username or alias; Microsoft does not allow removal of its only username.
3. Select Remove beside the phone number, then sign out.
4. Sign in to the account you currently use.
5. Open Manage how you sign in to Microsoft.
6. Select Add phone number, enter the complete corrected number, and verify it.
7. Make it primary if required, then remove the old number containing the missing digit.^1^

If Transfer number is unavailable

Microsoft indicates that the number was recently used on another account. Select Change number and use another mobile number for now. If this number is new to you, retry after 30 days, when the transfer option may become available.^1^

If you cannot access the account holding the number, use Contact Support from the same support article. Support can assist with sign-in troubleshooting, but agents cannot directly access or change account details.

References

1. Help with reusing phone numbers in a Microsoft account | Microsoft Support