Check for unknown apps on the Home Screen and in the App Library
Review Settings for unfamiliar profiles or device management under General > VPN & Device Management
Check for unknown VPNs under Settings > VPN
Look for unusual battery drain in Settings > Battery
Look for unusual data usage in Settings > Cellular
Check for unexpected pop-ups, redirects, or calendar spam
Review Safari for suspicious website data and extensions in Settings > Safari
Check for unknown email accounts, calendars, or subscriptions
Review installed certificates in Settings > General > About > Certificate Trust Settings
Make sure iOS is up to date in Settings > General > Software Update
Remove suspicious apps, profiles, VPNs, and certificates
Restart the iPhone
Change your Apple ID password
Enable two-factor authentication
If problems continue, back up important data and erase the iPhone using Settings > General > Transfer or Reset iPhone > Erase All Content and Settings