I, too, have an issue with data leaks. I'm using an iPhone 5 with iOS 6.0.4. I have been reluctant to go to iOS 7.
In any event, I got hit with overages while in Europe a couple of months ago and didn't realize it was a "leak" issue until I got back and almost immediately received a notice from AT&T that I was about to hit my domestic limit...after being home for 10 days. When the same type of thing happened in Nov, I checked my data usage more carefully. Turns out, my biggest days in Europe were the days I was at a friend's house and had wifi service...or so I thought.
Domestically, the spikes are random...day, night, morning. But when I check my calendar, the spikes frequently occur while I'm doing something else, like hiking in the woods or at my health club (where there's wifi anyway) or just not connected...or so I thought.
So I contacted AT&T as well as Apple and experienced what everyone else here seems to have experienced… stonewalling! Or suggestions that make it worthless to own an iPhone.
I don't have any solutions, but I at least wanted to post the reply I received from AT&T after several phone calls and email exchanges. (Their response is at the bottom of this message.). For the record, I do no streaming of music or radio, I am not attached to any hotspot or business server, almost all of my iPhone connectivity is in places where I thought I had Wi-Fi, I don't send diagnostic data to Apple or AT&T, and all the other switches that presumably allow your phone to connect in the background have been set to off...no apps automatically refreshing, no push notifications, not even icCoud backups.
While I'm no techie, it seems that the culprit is the phone/software itself. Apparently, it doesn't default to Wi-Fi when Wi-Fi is available, even if the phone shows the Wi-Fi symbol, and even if you think it's connected to Wi-Fi. Personally, I think the pressure needs to be put on Apple to fix the issue. However, it appears that short of some sort of class-action lawsuit, the problem will persist.
In the meantime, if anyone has any suggestions so that the phone can be used as intended, without incurring significant data charges, please reply.
JPHecht
What follows is a AT&T's response:
I have reviewed your email with management and there is no data leakage between AT&T and Apple.
In certain situations, data transmits over the AT&T data network even
when a device connects to a Wi-Fi Hotspot or personal/business Access
Point. In these situations, data charges accrue. Smartphone operating
systems (OS) behaviors and individual applications, among other factors,
can also impact data billing. Below are some examples of scenarios when
data charges may accrue even when connected to Wi-Fi.
1. AT&T applications such as, but not limited to, Address Book, Music,
and Radio.
2. A data session initiated at a point with no Wi-Fi connection.
Push notifications.
3. A Wi-Fi connection that cannot be re-established.
4. A brief delay in connecting to Wi-Fi after a device awakens from
sleep mode.
5. Many smartphones (including iPhone) are "always on" data devices.
These smartphone devices establish a data connection with AT&T's network and keep it open, even if no data transmits.
If a user does not end a data session, it records during a nightly feed
to the billing system.
A data session can contain billing for all the data transmitted
throughout the day.