Number sync device unknown

There is an iPhone mini syncing with my account 

I do not own nor did I set up number sync with an iPhone 12 mini. 

I want to know who’s phone it is, how it was linked to my account, why the “remove” option is unavailable for me to use to unsync this unknown device, what all personal information this device has gotten, how long it’s been synced to my account, I want to close my account, report thift, identify fraud, full reimbursement, new phones and phone plans and I want charges pressed.