It happened twice without an apparent connection, when I receive a call, the touchscreen is unresponsive and I can't take the call. The display is not frozen, because I can still see the call dropping and the physical buttons still work (volume, for example). But I cannot reboot the device, because I can't touch the selection "airplane/reboot/shutdown". I have to unplug the battery. Then everything comes back to normal. Any ideas how to prevent this or how to unfreeze the touchscreen?