Turn on Airplane mode
Disconnect from Wi-Fi and mobile data
Restart the phone in Safe Mode
Uninstall suspicious or recently installed apps
Remove apps with Device Admin access if needed
Clear browser data and download history
Delete suspicious files from Downloads and file manager
Run Google Play Protect scan
Update Android OS and all apps
Change passwords for important accounts from a clean device
Enable two-factor authentication on accounts
Back up important data
Factory reset the phone if the problem remains
Restore only trusted data and apps after reset