iPhone 6 has been displaying "No Service" an awful lot since I've had it. At work on my last phone would always have at least 2 bars of signal and has never ever dropped signal completely. My iPhone 6 does display 2 bars at work so I know the signal strength here is OK but only for about 50% of the time, the rest of the time it just says "No Service". I can usually get the signal back by toggling Aeroplane mode but that doesn't always work. Any fix for this?