I have an iPhone 4S and an iPhone 6, which have 400mb and 1.25gb data plans, respectively. Wi-fi assist is turned off on the iPhone 6, and the iPhone 4S does not support it in the first place. I reset the usage stats at the beginning of each billing month on both devices. The iPhone 6's reported cellular data usage is more or less identical to what my carrier reports; on the other hand, my iPhone 4S's reported usage is way off when compared to my carrier — as much as ~300mb off. Both devices are on wi-ifi nearly all day, and rarely more than 25–30 feet from an access point. I have no reason to believe that the wi-fi signal is weak in my house, as I never experience drop outs and have a very strong signal. In fact, I have two access points in my apartment to make sure there are no dead zones. I contacted my carrier about this and they claim there is nothing wrong on their end. Is there any way that the 4S could be using cellular data without me knowing it? At this point I just shut off cellular data on the 4S when I'm at home, as I've seen chunks of data get used up (according to my carrier's stats) even while it is on wi-fi. Neither device is jailbroken, so I have little reason to believe that malware could be at play.