How does Call Alerts work?
Call Alerts will let you know who’s calling even before your phone rings. This allows you to set the phone on silent if you want to ignore it or move to a quieter place if you want to speak.
This works only when a Truecaller user calls you. This uses the data / Wifi to securely send the alert from the caller's phone to yours, as soon as a call is made to you. Since data/Wifi is faster than a regular cellular network, the notification reaches you first, before the actual call comes through.
(Note: This features needs Notifications permission to work. You can disable Call Alerts anytime from the Settings section of the app.)