If you think your number has been used by someone, you can directly contact Microsoft since they are the one who has an access to your account.
You may try the following steps:
- Kindly go to https://support.microsoft.com/en-us/topic/talk-...
- Click on Talk to an agent.
- Once connected, Select Microsoft Account
You'd be able to talk to a person a few moments.
You can also contact them via phone:
https://support.microsoft.com/en-us/help/405170...
Please note that it will take time to be in line due to COVID-19 situation and your patience will be highly appreciated.
Thank you