This has been a consistent iPhone issue for me since iOS 7 on my 5. When I change locations sometimes my phone's message app just completely fails. This doesn't mean crashing but in addition, through settings ----> Messages, the iMessage button is completely unresponsive as well as FaceTime. You literally cannot toggle the button back to on. Restarting your phone and resetting Network Settings does no good. Anyone have a fix for this because this is so annoying.