There is no unique answer for this because it will depend on the phone settings.
The following cases describe most of the scenarios:
1. Alert call (beta)
*Only for stations with incident phone-calls enabled.
If your device supports simultaneous calls, the call from FSR will appear in the screen, your device might vibrate only, but there will be no ringing sound. You will be able to take the second call while the first one is on hold.
2. Push notification
a) If your phone is in silent mode, the notification will appear on the screen (depending on the configuration, your device might vibrate)
b) If your phone has sound on, the notification will appear on the screen, and the volume of the call decreases slightly.