By default, pressing the Power and Volume Up buttons simultaneously will put the Teracube into “Vibrate” mode for all notifications. If these buttons are pressed accidentally, the phone may stop ringing unexpectedly. Some people might like this shortcut - some may not.
To change this behavior, go to Settings > Sound and select “Shortcut to prevent ringing”. Tap “Press Power & Volume Up together”, then select a new behavior - either “Mute” or “Do nothing”.