I had the same problem as everyone else with my daughter's iPhone 5c using 9 GB of data in a month with normal usage.
We were able to fix this and get her data usage back down to 2 GB like the rest of us....
1) Apple released a fix for the iPhone to fix using Data vs. WiFi.
Check for phone UPDATES and make sure that it is Up-To-Date.
2) Verizon changed the Backup Plus (2GB) to Verizon Cloud backup with 25GB free.
Make sure that you configure for the Cloud storage.
Most of the data usage that I was seeing was a bulk at a time which indicated a sync.
This could be storage syncing to iCloud (2GB Free) or to Verizon Backup (moving to cloud fixes this)
iCloud users may need to buy more storage or delete old backup especially if you upgraded from another iPhone.
Hope this helps some of you out.
Sorry that I am not an expert and use an Android Phone.
(Use for home/work/nav. and only use 1.5 GB per month)
Apple was the leader in TECH, but lately they lead in $$$ and data usage....
Good Luck.