In recent times, we are seeing a phone number is verified using a missed call given on clients mobile number and it shows phone number is verified. How does this process happens?
A broadcast receiver (receiver) is an Android component which allows you to register for system or application events. All registered receivers for an event are notified by the Android runtime once this event happens.
So, the app which you are using for verification register a broadcast receiver for (on call recieve) call and as soon as it happens, the android system notify the app.
More infos on google developer
Here is an example