This seems to be happening a lot lately. You’re gonna need to go into your settings > go down to calendars, > click on accounts. And that’s where you’ll probably see some emails logged in that you don’t recognize. You’re gonna need to remove them. I have attached some links on how to do that. And then also a link letting you know exactly what just happened. It’s called phishing,
Delete spam calendars and events on iPhone
https://support.apple.com/en-us/HT211076
Recognize and avoid phishing messages, phony support calls, and other scams
https://support.apple.com/en-us/HT204759