Someone else is using my phone number for their microsoft account.

Because the phone number is already associated with another Microsoft account, it behaves as that account’s sign-in alias. When the password was reset using that phone number, the password for that existing account was changed, which is why a different user’s account appeared.

To address this, follow these steps:

1. Stop using that phone number to sign in right now
   Do not continue accessing the other person’s account. Sign out of the account immediately on all browsers/devices.
2. Check whether the number is already an alias on a Microsoft account actually owned
   If there is a Microsoft account that belongs to the rightful owner of the number, try signing in with any known email address for that account and then add or manage aliases from there. Microsoft treats email addresses and phone numbers as aliases that can be used to sign in and share the same account data. See the explanation of aliases here:
   “Email addresses and phone numbers associated with your Microsoft account are called aliases… You can use any of your aliases to sign in to Microsoft services… Your aliases share a single password.”
3. Use the built‑in transfer/remove flow if the UI offers it
   When trying to use the phone number again, Microsoft may show one of these messages: “[phone number] is already a Microsoft account”
   In that case:
   • If the number is associated with a different Microsoft account and should be moved to a new account, select Sign up from the message and follow the prompts to remove it from the existing account and move it to the correct one.
   “We noticed [phone number] is already used to sign in to [another number]”
   In that case:
   • If the number is an alias for another account that is actually owned, select Continue and sign in to that account.
   • If the number is an alias for a different account and needs to be associated with the correct account, select Transfer number and follow the prompts.
   • If Transfer number does not appear (for example, because the number was recently used on another account), select Change number and try again after 30 days to see if the transfer option becomes available.
4. If the number is new and clearly should not belong to that other account
   If the phone number was recently assigned by a carrier and clearly no longer belongs to the other person, the system may still treat it as theirs for a period. In that case, the transfer option may only appear after some time (Microsoft notes that if the phone number is new, try again after 30 days to see the Transfer option).
5. Secure the actual Microsoft account and phone
   • Ensure the real Microsoft account (the one that should be used with Minecraft) is secured with correct security info and two‑step verification if possible.
   • If any unusual sign‑in alerts or verification codes are received that were not requested, review account security and recent activity as described in the unusual sign‑in and verification‑code guidance.

If none of the transfer/remove options appear and the system continues to treat the number as belonging to someone else’s account, the only supported path is to follow the on‑screen flows (including waiting for the 30‑day window) and use a different phone number as security info in the meantime.

References:

• Help with "phone number is already used" in a Microsoft account
• Why is Microsoft texting me?
• Troubleshoot Microsoft verification code issues
• What happens if there's an unusual sign-in to your account
• How to recover a hacked or compromised Microsoft account