Hacking
Received an alert on my calendar that my phone might have a virus or has been recently hacked. Could this be true
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.