Who I have the same questioned this topic

This seems to be a common problem. My daughter is using 40MB to 150MB 4 times a day - every 6 hours.

My wife and I don't have this problem. All 3 of us are up to date on iOS. So, my guess is a rogue app.

It uses the cellular connection even if the phone is connected to WiFi. Blocking cellular access works. But, that requires turning cellular on when needed and remembering to turn it off when done.

The usage summary on the iPhone does not show this rogue data.

Does iOS have a log of data usage (not on the Verizon web site, on the phone)?

How do I scan the phone for rogue software/malware?