I was having the same issue. I learned that why you are on Wi-Fi you should turn your cellular data off. According to an AT&T Rep, your Wi-FI is shut off when you phone is asleep. So if you have apps open then the data will pull from you're cellular data.
There must be a bug in either AT&T or apple software. You still should call and complain, this the only way the companies realize there is a problem.