my phone number is being used by another Microsoft account

Question

My phone number is being used by another Microsoft account. I am therefore worried about the security of my account.

Answer 1

When a mobile number is already associated with another Microsoft account, it does not by itself give that other account access to data or services in a different account. However, it is important to ensure that the number is correctly associated with the intended account and not left on an account that does not belong to the rightful owner.

To address this:

1. Determine how the number is being used
   • If the message says “[phone number] is already a Microsoft account” when trying to create a new account, that phone number is already the username/alias of an existing Microsoft account.
   • If the message says “We noticed [phone number] is already used to sign in to [another number]” or “This phone number has already been taken” when adding it as an alias, the number is already an alias on another Microsoft account.
2. If both accounts belong to the same person and the number should move to a new account
   • When creating a new account and seeing “[phone number] is already a Microsoft account”, choose Sign up from the message and follow the prompts to remove the number from the existing account and move it to the new one.
   • When adding the number as an alias and seeing “We noticed [phone number] is already used to sign in…”, choose Transfer number (if available) and follow the prompts to move it.
   • If the Transfer number option does not appear, the number was recently used on another account. Use Change number for now and try again after 30 days.
3. If signed in to the account that currently has the number and it needs to be moved
   • Go to the account that currently has the phone number.
   • Remove the phone number from that account:
     1. Sign in to the account that has the phone number.
     2. Go to the names/aliases management page.
     3. Select Remove next to the phone number.
        • The phone number can only be removed if there is another username on the account, and it is recommended to have at least two additional forms of contact on the account.
   • Sign out, then sign in to the account where the number should be used and add it there as a phone number.
4. If the number is on an account that does not belong to the rightful owner
   • Do not attempt to access the other account.
   • Use a different phone number or email address for the current account’s security info.
   • For sign-in to Microsoft services (including Skype, OneDrive, Xbox, Office), if the email or phone being added triggers “Microsoft account already exists. Enter another email or phone or get a new Outlook email.”, use an alternate phone number or email, or create a new one.
5. Understand aliases and security
   • Email addresses and phone numbers on a Microsoft account are aliases that share the same password and access to the same data for that account.
   • Any alias can be used to sign in, but having a phone number as an alias on another account does not grant that other account access to a separate account.
   • For security, ensure:
     • The phone number is only on accounts that are controlled by the rightful owner.
     • There are multiple recovery options (additional email addresses or phone numbers) on the primary account.

If sign-in or account ownership is in doubt, use the guided tool linked in the aliases article to help find the correct account and recovery path.

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References:

• Help with "phone number is already used" in a Microsoft account
• I received a "This phone number has already been taken" message
• Why do I get the message "Microsoft account already exists" when signing into Microsoft services with my account for Skype?