BB Bold :: 9900 : Display Gets Dark During Call?

blackberry bold 9900: on any call, the display dims down within some seconds, all keys and the touch screen are disabled except the "end call" key. I need the "mute" and "loudspeaker" functions which are usually displayed on the screen during a call, So they are displayed on mine, but they are disfunctional and vanish when the display dims dark.Even if I put the blackberry on the desktop and keep 0.5m distance from it, it dims to dark screen within 2 seconds after pressing the "call" key, and even an immediate press on the "loudspeaker" icon in the left bottom corner of the touchscreen within these 2 seconds is ignored.