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Make your phone play a sound (notification ringtone) or vibrate when you hangup or when the line is disconnected.
* Additional sounds can usually be saved in /sdcard/media/notifications/.
* Low vibration intensity (or none) on SAMSUNG devices: Go to Android's System Settings, Sound, Vibration Intensity and set the intensity for Haptic Feedback. You don't need to enable haptic feedback, just adjust the intensity setting. This value appears to be used on some devices even for a non-haptic feedback vibrate.
* Please email problems.
* Add read storage permission to avoid problems when user-added sounds are selected in Android 4.4+.