My calendar keeps saying phone is hacked

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

You don't have a virus, but you did click on a link which downloaded calendar spam to your phone.

See this link to remove the calendar spam --> Delete spam calendars and events on iPhone - Apple Support

Be careful clicking links from sites you don't know.

It is a scam. Try this:

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