Getting a message "your iPhone is damaged" in my calendar

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

Hi there! Thanks for asking the community!

I am sorry to hear that you're experiencing this issue. Sometimes, what scammers will do is try to get you to subscribe to a calendar, in which they will add events which try to scare users into thinking their phone has an issue.

Often times, they will use wording such as "Your phone is INFECTED, protect your phone NOW!!!!!" They usually target older users, who are more susceptible to scams like this.

How to fix it? Just go to your calendar settings on your iPhone, and remove the subscribed calendar. This link below shows more information about multiple calendars on iOS.

https://support.apple.com/guide/iphone/use-multiple-calendars-iph3d1110d4/ios

Hope this helped!