I've had my 4s a couple of weeks now and from day 1 I've had this problem. The problem being that every now and then when I make a call, although the call is made there is no ringing sound and when the person I'm calling answers neither they nor me can hear anything. It happenened 3 times today which is the most it has ever happened, most days go by and there are no issues. Do you think I've got a dud 4s? Will apple replace this phone if I explain the problem to them?