This issue is affecting others as well. There is a thread on the Vodafone website as well detailing your exact problem. It all started after the latest software update. I would contact HTC and notify them of your problem, as many of us have done, also push O2 to contact them as well to get an update. The more they know about it and the more people who put pressure on them, the quicker it will be resolved. I contacted HTC and they said suggested getting a new SIM card, and then a factory reset. None worked.
http://forum.vodafone.co.uk/t5/HTC-One-Series/HTC-One-S-Stuck-in-Airplane-Mode-when-Turning-on/m-p/1...
This is a copy of someone else's posting that details the issue:
have not used Airplane mode for months.
I don't have any pin or other security on turning on or waking the phone.
I have always switched it OFF when I go to bed.
I switch it ON when I get up in the morning.
Yesterday it took me a while (I don't function too well in the morning) to realise that the phone was showing no signal on the notifications bar but the WiFi was working fine so I wasn't bothered. When I realised it wasn't a local Vodafone fault (no people on Facebook moaning) I started playing with the phone.
On turning off it seems the phone gets stuck trying to switch OFF Airplane mode and stays this way when turning on. The Airplane mode toggle is greyed out and won't work under settings and the Airplane mode option is greyed out and unaccessible on the menu when pressing the power button. It also says there "Switching OFF Airplane mode......" (or very similar).
As I said earlier I've not used the Airplane mode for a long while.
Doing a "Restart" from the power button menu seems to clear it then.
So why isn't the phone doing a "Restart" from being off?
This would seem to be the question which needs answering.
During the restart it does flash a notice on the notification bar about "Removing" a "Restriction" (or similar) but it doesn't hang around to be read.
Good luck and keep pushing them.
