Every day, data goes out from his iPhone at the same time (3,6,9 pm). Sometimes it is a small amount, sometimes it's 300 MB. What gives? Leo says what AT&T is doing is listing the data in "chunks", not what is actually happening, If it was minute by minute, Bill's bill would be 50 pages. Most likely the phone is "phoning home" to various apps, doing app updates, etc. Here's why it does it - https://forums.att.com/t5/Wireless-Account/Data-Charges-every-3-hours-12-3-6-9-even-when-phone-is-off/m-p/4921313#M236519
But why doesn't it use wifi to do it? Leo says he can control that by saying which apps can use cellular data or not. That way, when he's out and about, those apps can't work until they have WiFi. Look in the cellular settings.