erm dont really get your issue here. disconnected from the line when you are on the phone or when the phone on stand by mode?

if on the phone mayb you accidently click it with your cheeks. this happened to me before cos the heat sensor didnt sense my hear. but that was just once or twice.

and for disconnection during stand by mode, perhaps your simcard slot got problem? try using simcard from other service provider? if problem still persist, then its probably your simcard tray loose already.