My iPhone has been hijacked
My iPhone has been hijacked. The screen flashes a message to call 1-870-259-4849. I am reluctant to call but phone is rendered useless. It can't be turned off. Removal of SIM card does not help. I finally call. The person answering claims to be with Apple Support. They ask for the model of phone which I give. They then ask for my email address which I do not give.
Please advise.
Thanks
iPhone 11 Pro Max, iOS 14