Toggle Airplane mode.
To restart your device, press and hold the sleep/wake button. After a second or two, the slider will appear. Drag it, and turn off your device. Again, press and hold the same button, and turn on your iPhone 6.
Update your device, because that will fix the problem, and some other glitches as well, if you have any.
To check if any carrier-updates are available, click on Settings, tap on General, tap on About.
Back up your files to iCloud. Connect your iPhone 6 to your computer with the USB cable. Select your iPhone once it appears in iTunes. Choose Restore from the Summary panel. Once the device is restored to its factory settings, follow the steps to set up your phone as new.