Restart the iPhone
Update iOS to the latest version
Delete suspicious apps
Clear Safari history and website data
Remove unknown calendar subscriptions
Remove suspicious profiles or device management settings
Check and delete suspicious VPNs
Reset network settings
Restore the iPhone from a recent backup
Erase all content and settings if the problem continues
Change Apple ID password
Change passwords for important accounts
Enable two-factor authentication
Avoid installing apps from unknown sources
Keep iOS and apps updated