sudden and unexplained increase in data usage

For no apparent reason, one of the iphones on my account is using an inexplicable amount of data each month. I originally assumed the fault lied with the user but after turning off almost everything that can use data, the data usage continues. I spent three hours on the phone with Verizon today who suggested that I increase to an unlimited data plan. I found an article in Forbes where Apple apparently is aware of this issue but is not able to comment on it. Whatever that means?!!>!>

Has anyone else had this issue and discovered a solution to fix it? I am not willing to give Verizon more money for my cell phone bill, which is almost as much as a car payment at this point.