Completely disabling cellular data and limiting apps only to WiFi when available I've recently (2 months ago) purchased my first smartphone. I'm on a sort of prepaid contract (I pay what I use in terms of calls and SMS and it's automatically withdrawn from my bank account).

For years I have been paying like 6-7 EUR per month (at the most!). I don't like using phones in general and smartphone is a new thing for me but I bought it just for the cheap GPS and camera (not the mobile surfing). After the first month I got a surprising 30 EUR bill. Every once in a while I was getting notifications about new mail, or Skype messages etc. and I was wondering why this was happening (I know, I sound like a member of the Flintstone family) because I never enabled mobile data transfer. As it seems it is turned by default.

I went to my service provider and the guy deactivated the data connection. To my question "Can I just block any Internet activity through cellular connection for my number?" he told me that it's not possible (why he did not explain). Just to make sure things were all set I even put a data cap on cellular data at 10KB. In addition I have also limited updates of apps only to WiFi that is if I'm connected to a WiFi network only then apps should check for updates and download those.

I've just received my bill for last month (the second month of me using the smartphone). And this time it was just painful - 81 EUR. I checked my bill's details and I saw the following strange reoccurring Internet traffic:

Data usage
Date       Time     Service  Units Cost
25.04.2018 04:23:28 Internet 38:54 Min 2,9410
26.04.2018 14:35:54 Internet 38:54 Min 2,9410
27.04.2018 16:59:36 Internet 38:54 Min 2,9410
28.04.2018 04:53:32 Internet 38:54 Min 2,9410
29.04.2018 02:37:58 Internet 38:54 Min 2,9410
30.04.2018 11:23:06 Internet 38:54 Min 2,9410
03.05.2018 13:57:57 Internet 38:54 Min 2,9410
04.05.2018 12:43:26 Internet 38:54 Min 2,9410
08.05.2018 05:08:03 Internet 38:54 Min 2,9410
09.05.2018 15:13:30 Internet 39:34 Min 2,9410
13.05.2018 19:34:04 Internet 1 Min 0,0756
13.05.2018 23:22:19 Internet 37:58 Min 2,8654
14.05.2018 14:11:45 Internet 38:54 Min 2,9410
15.05.2018 10:21:09 Internet 38:54 Min 2,9410
16.05.2018 10:32:32 Internet 38:54 Min 2,9410
17.05.2018 10:23:51 Internet 39:23 Min 2,9410
18.05.2018 19:38:51 Internet 38:54 Min 2,9410
19.05.2018 09:48:12 Internet 38:54 Min 2,9410
20.05.2018 04:41:42 Internet 39:34 Min 2,9410
21.05.2018 06:16:51 Internet 39:34 Min 2,9410
22.05.2018 10:05:11 Internet 39 Min 2,9410


All in all 68 EUR without VAT, 81 EUR with it.

Needless to say I am far from happy.

I started digging and checked the data use for every single app. Almost all apps had the background data use (cellular connection) enabled by default. I wan't sure what the background data is so I read a little bit.  The descriptions is also useful namely: Enable usage of cellular data in the background.

I checked some of the apps (like YouTube) and I saw that while 80% or so of the traffic was foreground many times there were couple of hundreds of megabytes background data.

I disabled all background data use now for all apps but I wonder if that is not the case why I got that fat bill. If that's the case there are multiple things that bother me namely


why did the cellular data limit not kick in? In the stats for cellular data I still get literally no data being used
why even with data connection disabled cellular is used at all?
why am I not getting Skype, Hangouts etc. messages, new email notifications, Chrome shows I'm offline etc. if cellular  is indeed enabled?


Especially with Google Photos (I have syncing between my phone and the online service enabled) the situation is horrible - 466MB foreground, 2.08GB background between 10th of May and 7th of June.

What else am I missing? Clearly the guy at the shop didn't help and I'm very new to this smartphone thing. I simply want to use Internet (any sort of Internet connectivity) ONLY when I'm connected to a WiFi network (home, work).
First, "not possible" is a straight lie. The carrier is perfectly capable of turning that feature off and limit you to "calls and SMS only". They simply don't want to.

Second, you could try some Firewall app in whitelist mode (i.e. by default all apps are blacklisted). Take e.g. a look at NetGuard (I recommend taking it from F-Droid, though it's also available at Playstore). This firewall app doesn't require rooting your device, and you can set up mobile-data restrictions separate from WiFi:

  
NetGuard: List of apps, settings for a specific app, defaults (source: Google Play; click images for larger variants)

As the screenshots show, you can set the default to "block mobile by default", so you don't have to take care for new apps unless you want to allow them.

Third: I find it very strange that mobile data are summing up if you have disabled them, that shouldn't happen. But using a firewall app, you should at least be able to reduce that massively.