Crazy "System" traffic data - What to do? ios 9 - iphone 6s I have a very limited data amount (only 1 Gb of 3G / 4G) per month

So natrually I took all the steps in order to limit my data drainage:


Disable Wi-Fi assist
Only five applications have access to cellular data traffic. Whatsapp, Messenger, Safari, google maps, and Mail. That's it.


Nonetheless, over the last 4 days, my data consumption was 394 Mb, apparently!!
It's out of the question I used them all with the aforementioned application.

The only odd thing is "System Services" which counts for 460 MB! In that, I have a "General" amount equal to 359 Mb, 15.6 Mb of game center (! I never use it!) and 36 Mb of iTunes. 

I want to disable all of that! What to do? Can somebody help?
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.