Hacker Threats

If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

The Apple support article is below. 

https://support.apple.com/en-us/HT211076

Your phone can’t be hacked; those messages are scams from criminals hoping that you will click on a link and answer questions that will allow them to steal your money or your identity.

You got tricked into installing a rogue calendar; your phone does not have a virus.

If your phone has iOS 13 or earlier: Go to Settings/Passwords & Accounts, look under Subscribed Calendars, tap on the offending calendar and delete it.

If your phone has iOS 14: Go to Settings/Calendar/Accounts, Tap Subscribed Calendars, tap on the offending calendar and delete it.

And in the future beware of "free" offers