Tap on Settings, Bluetooth, and turn it off. After a couple of seconds, turn it back on again.
Press and hold the sleep/wake button, and drag the slider in order to turn off your iPhone 6 Plus. When it’s off, press the same button, and turn the device back on.
Turn the device off and then on again. As you know, restart often solves many bugs, and Bluetooth device is no exception, so maybe this solution will work out for you.
This is a very simple fix, and also a very effective one. Navigate to Settings, tap on General, Reset, and choose Reset Network Settings. Hopefully, the problem will be gone.
Back up your files to iCloud, connect the phone to the power source, and a Wi-Fi network. Click on Settings, General, Software Update. Choose Download and Install.