The HTC Touch Pro 2 is among the very few Windows Mobile device that has a proximity sensor (the other being the LG Incite). The purpose for the proximity sensor is to turn off the display when you’re on a call to conserve battery. Then, when you pull the phone away from your face, the screen goes back on so that you can view you calendar, add another caller, etc.
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Proximity sensors are absolutely essential, for organisations that use large, heavy vehicles – when used for reversing. However, they are also very useful for smaller vehicles, though they can manage without them. In both cases, accidents are substantially reduced.