Jim has an iPhone 5 through Verizon and he's being nailed for "excessive use of data" with charges over $300 a month, and he has no idea why because nothing has changed in his usage. Verizon claims he's using up to 1 GB a day! But he isn't using even close to that. He doesn't stream video, and is rarely listening to Pandora. Leo says that going into the usage meter in the settings, he can see what the phone thinks he's using. Is the hotspot turned on? Streaming video can easily get 1GB a day if he's not careful. And since he can't receive phone calls, that tells Leo that data is constantly on. Try leaving the phone off for a day, or put it in airplane mode and see if there's any data usage. If there is, then it isn't the phone.
Leo recommends doing a factory reset and leaving it in a pristine Apple state and see how the data runs. It could be a rogue app that's doing it. He can also go into the settings and look at individual app usage. Go to Settings > Cellular > then scroll down to the apps, and he'll see the usage broken down by app. It's also possible that the ESN number of the phone has been cloned and someone else is using his data. Leo recommends calling Verizon to ask to change the ESN or the Data SIM.