I've been having this issue for a week, i initially thought it was a factory defect, as it only occurred when my phone was was online, sent it to apple, they couldn't fix it (as they also thought it was hardware), they told me to change sims, and it worked for about 18 hours before messing up again, and it worked again after my phone died and i rebooted but, after i tried to change some settings, it messed up again. I have sprint, and it worked for 18 hours with a T-Mobile sim, and then my iPhone started to reboot, and there was some issues after that. Its an iPhone 7