Strange. The only thing I can think of is to try and reset network settings. Was it a used iPhone? If by an off chance it was a certified pre-owned device (CPO) and is under warranty definitely have Apple take a look at it. Here are the steps. It does erase your saved Wi-Fi networks but it is something Apple will request you do. If this doesn't work, back up the device and then restore it again.... its something Apple requires.. and that is all I can think of.