Crazy "System" traffic data - What to do? ios 9 - iphone 6s
I would look at which Apps are allowed to use Background App Refresh, in Settings > General &amp; also possibly in Settings > iTunes &amp; App Store what is allowed to download over Mobile data.

A third possibility is iOS updates themselves, which cannot be prevented; the best workaround for those is to always update on the day of release, through iTunes, rather than let the update download to the phone.