1.Intent.action == Intent.CALL_ACTION is true when a phone call is received on the device.
2.Intent.action = Intent.CALL_ACTION is used when a phone number is to be dialled without showing a UI on the device.
3.Intent.action = Intent.DIAL_ACTION is used when a phone number is to be dialled without showing a UI on the device.
4.Intent.action = Intent.CALL_ACTION is used when a phone number is to be dialled without the user having to explicitly initiate the call.