I have the same problem; tried a phone restore, both restoring data and setting up as new phone with factory settings, reset all settings, re-boot, remove apostrophe, put in fridge, you name it!
I think there's quite a few iphone 4s users with the same problem!
I called Apple they said it's a hardware problem, but it has worked for nearly two years; they say go to the Apple store and they can provide a replacement phone for approx. £150!
I've been into the O2 shop where I bought it and they knew the problem exactly; said they had sent a few phones off for repair and had them back awith a report they cannot fix it because it's a software problem!
An assistant in the shop had the same issues; logic says it's a software problem and hopefully Apple will fix it with a patch but at the moment they seem to be avoiding everyones comments!
If anyone else has any ideas these would be appreciated?