Does the iPhone data usage counter pretty much agree with the carrier?
No - This is rare, but it's happened. If the carrier is reporting massive data usage that your iPhone isn't seeing, it's possible that there's mis-reporting happening on the carrier end.
Yes - They pretty much agree: Then your phone is really consuming that data, and you have to isolate the app or process. Causes can be any of the following: Message stuck in Outbox on an email account, Cellular Data being selected for iCloud functions including backup, document sync etc, iTunes Match, streaming audio and letting the phone fall asleep (which causes it to switch from WiFi to 3G), sync issues with email/calendar account, the list goes on.
The large data hit on the carrier report is usually a subtotal of most recent usage, not a timestamp of exactly when the usage actually occurred.
Isolating excessive data takes patience and some detective work, but it can be done.
AT&T always says "Sent" regardless of direction. On the iPhone counter, you should add sent+received to compare to AT&T's numbers.
If Sent+Recieved on the iPhone = 500MB, it's something on the phone. Maybe it fell alseep while streaming Pandora or something. It doesn't stay on WiFi once the screen goes dark, unless connected to power - and I've seen cases where it doesn't even stay connected to WiFi then.
If Sent+Recieved on the iPhone = 10MB, and AT&T claims 500MB, then that's a possible indicator of carrier misreporting. Getting a carrier to admit that is almost impossible, but Apple engineering seems quite confident that the iOS numbers can be trusted within a few percentage margin of error.
Kkashyyyk - just curious - do you have an AT&T Microcell at home?
We also have 4 iphones. Only two have this problem. In my case, I know it's the Microcell but that doesn't mean the same thing couldn't happen with a tower, and it doesn't explain WHY the data is being over-reported.
Ours began consuming data March 13th according to AT&T, so we're nearing the end of month #2. I still don't have a solid answer as to why. I have a case with AT&T, but that's no assurance I'll ever get an answer. I continue to work on the issue - I always assume I'm on my own when troubleshooting stuff like this. I will post back in the days ahead as I narrow things down more. Meanwhile, to ease the pain, if you have WiFi at home, I'd suggest turning Cellular Data off on the iPhone when you're around the house.
I am having this problem too on an Iphone 4. It has been going over on the data plan for the past 2 months. When the new billing cycle started, my daughter turned her data back on and it was over on the data within a week. She turned the data off again but we just recieved a warning from ATT that even with it off, the phone is about to go over the next data level. We have 3 other iphones that are not having this problem.