Here in the UK, I'm on my second Iphone 5. Took the first one back as Apple, O2 and Phones4U told me it was probably a hardware problem with the wifi. Well guess what? Brand new hand set, same old problem. I can force it to join wifi at home and at work (did this by manually entering connection codes as 'Other') so at least I'm not using data from my monthly plan up there. But on the commute in between (thankfully i have no life otherwise, sigh) I am finding that the phone is racking up HUGE data usage. And I mean absolutely gigantic. In fact it's used more in 4 days than my Iphone 4s used in 3 months. By the end of day 1 with my new phone I'd used 1/2 my monthly data plan, simple by opening an email. Utterly ridiculous.
I've been monitoring data usage every few minutes over the past few days and this is what I've discovered. When cellular is off, no transmission of data occurs. When I'm on wifi, no transmission of data occurs. When mobile data is on, but 3G is off, no leak occurs. Switch on 3G, however, and the data sent/recieved starts rocketing up. Even with all background apps closed. Even on sleep mode, in my pocket, doing nothing.
So the problem can only be with the way the phone responds to having 3G selected to 'on'.
It seems when 3G is on, it is constantly communicating with the servers, whether you've opened any apps, whether you've chosen to or not. To be clear: All notifications are off. Email push is off. Icloud is deleted (which reduced data usage massively. If you still have icloud get RID of it). Neither Itunes or anything else is selected to pull data. Also gmail, deleted. (also helped massively). Location services off. Everything, even ad tracking is off. But it still pulls data. Where from?!! It seems when 3G is on it is constantly trading data with the server, racking up huge amounts of data sent and recieved.
Happens with my colleague's 4S on IOS6.01 too. The problem is not the handset, it's the operating system.
IOS6 is the biggest failure yet for Apple, buggy and awful. (not to mention how terrible maps are). If Apple don't fix this MASSIVE problem soon i think a legal worldwide class action case can't be far off. I'd support it. If you pay £400 for a handset you expect it to work, not to be so afraid of switching 3G on that you feel unable too. the phone is useless without email push or internet connection. It's the opposite of a smart phone. I might as well ressurect my old Nokia.
Apple needs to sort this out right now.
I've been a lifelong Apple fan but after his fiasco 'll NEVER buy Apple again. IOS6 is an absolute disaster.
Patch It, fix it, whatever, just sort it out apple. Pretending it's not happening is not going to make this go away.
As for this phone? Until I see a fix, it's going in the bin.