Someone else’s mobile number has appeared in my Apple ID

You might want to consider changing your password and making sure it is a strong one. You can also add 2 Factor Authentication to your account for more security. If the attempts continue, you might want to consider a name change.

Apple ID Name Change

Apple ID Password Change

What to do after you change your Apple ID or password - Apple Support I suggest doing this before you change your Apple ID or password.

Apple ID -Two-factor authentication

Two Factor Authentification availability

Apple ID - Using app-specific passwords

Apple ID - Two Step Verification App Specific Passwords

Apple ID - Two Step Verifications FAQ