IS THERE AN OPTION TO SELECT TO KEEP THE SCREEN FROM GOING BLACK WHILE ON A CALL?  DOES ANYONE ELSE HAVE PROBLEM WITH PERSON ON THE OTHER END ALL OF A SUDDEN NOT BEING ABLE TO HEAR?  MY PHONE CALL GOES MUTE VERY OFTEN AND THE CALLER CAN'T HEAR ME--I CAN HEAR THEM BUT THEY CAN'T HEAR ME.