Unknown IMessage number
An unknown random telephone number has managed to be approved to access my iMessage and Facetime accounts. How do I deregister this number if I don’t have access to the phone or SIM card?
iPhone 8 Plus
Does the number show on your Apple ID?
Have you tried changing your Apple ID password?
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.