Go to Settings, turn on Airplane mode, and turn it off again.
Press and hold the sleep/wake button for a second or two. The slider will appear, and once that happens, drag it, and turn off your device. Turn it back on.
Tap on Settings, click on General, and then About. If there are updates, download and install them.
If there is a new iOS version available, make sure to update your device. It may fix this problem, and some other glitches you might have with your iPhone 6.
Back up your files to iCloud first. Navigate to Settings > General > Reset > Erase all Content and Settings. This may last for several minutes, or an hour or more. Once the task is done, set up your device as new.