Should the Voice Responder become unresponsive or show any other strange behavior, you can first try a soft-reset by pressing both PTT buttons and both arrow buttons until the LED turns off. Then press the Emergency button followed by the power button until the device turns off. Let go the power and then the Emergency button.
To hard reset when the device is on, press both arrow buttons (<) and (>), Emergency and power button at the same time. This should only be done when other ways have not worked as it is harsh on the processor.