Go to Settings, and turn on the Airplane mode. After ten seconds, turn it off again.
By pressing and holding the sleep/wake button, and dragging the slider when appears, turn your device off completely. Leave it that way for several minutes, and then turn it back on.
If you installed some new app on your iPhone recently, maybe it is the reason why the phone has this glitch. Remove the app, but tapping on its icon on the home screen. When an X shows up in the corner, click on it, and the app will be deleted.
Even If this sounds crazy, many users find this solution extremely helpful. Give it a shot.
Back up your data before you start. To do this, go to Settings, tap on General, click on Reset, and click on Erase all content and settings.