Talked w. an ATT tech support person, on 27 Oct, 2016.
My data plan keeps going up and over for no apparent reason, so she had me go thru these things on my phone, I followed her advice.
First, Go into Settings, Privacy, and turn off Location Services on the apps you won't need them for.
Second, Settings, Time and Location, also turn it off.
Third, What I already had done, but you need this on, if you are away from your Wi-Fi.
Settings, turn off all Cellular Data. Turn it back on when you leave the house.
Fourth, Hit home button twice, and swipe all these apps up so they go away, and stop running in the background.
These are ones that I already had turned off.
1. Cellular data, turn OFF all apps that you won't need like your photo apps. Leave on the maps and weather apps.
2. Settings, turn OFF Background refresh.
About 2 years ago, was when this issue appeared. And I had had iPhones for many years before that particular time. We figured it was in an iOS update, as that is when other people noticed it also.
I had to repeatedly call ATT and explain that I was always on Wi-Fi, and this was not supposed to happen. The phone was OFF and sitting in my pocket when I was away from home, except for occasional checking weather, using the phone, text, or other quick Safari searches.
They always deleted my $15 charge, said that this had been an 'issue' since an iOS upgrade. I forgot the exact date of it. Explained that being old and retired, that I am home most of the time and always have the Wi-Fi on. Do have the phone with me walking the dog or around town of course, but have turned most everything off as written above.
I/we never had this problem prior to that 2 year ago iOS update.
I have friends with this same problem, and would love to know how to really get rid of this problem.
I bet you have more info and help on this.