At least once a day, for the past 3 days, my iPhone has been suddently displaying "no service" within a pretty well covered area. It does not find signal again. In order to aide it in a search, i flip on and off airplane mode. This triggers a sim error popup message and "invalid sim" shows up at the top left of the screen. I need to reboot the phone in order to gain cellular service again. Has anyone had this problem before?