One thing I was thinking of that you might want to check;
If you set the device to aeroplane mode, deoes the same thing still happen?
At a dutch website there is an article about high data bills even when roaming was disabled. Even though they were not able to fully get the cause there might be some leads in why you get data bills
Tweakers article (original in dutch)
Tweakers article (in english by google translate)
I traveled quite a lot with my SFOS devices (both JP1 and XA2) and my wife has Android and we never faced that.
In Germany, providers do not recommend using a smartphone without a contract with a corresponding data option. It is also possible to have the provider set up an internet block here. This means that you are on the safe side if the Prider wants to bill you for allegedly used data. In the forums of the providers there are many complaints where mobile data was billed although the use in the smartphone was probably deactivated.
Maybe that would be an Option for you.
I got answer from provider’s support. They said that devices can generate traffic even when cellural data is off - for some system software updates.
Luckely they return my money back.
My logs made with ifconfig -a show zero traffic on rmnet interfaces, provider’s reports still show small packet like 1 Kb.
I don’t know who is responsible for that issue - OS or provider. But I want to remind that in my case WiFi was on so any system traffic could use wlan interface.
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I guess that in theory even the modem could autonomously transfer some data, since it’s basically a whole other computer in there. I wonder if “disabling cellular data” is purely OS-level, or if it also instructs the modem to do so. The latter case would be especially bad if it was indeed the modem “phoning home”.
Out of curiosity, is there any difference if you set it to prefer 3G or 2G? 4G after all is technically all just “data”, i.e. packet switched traffic.
On a side note i feel sorry for you when it is a significant saving to have such a terrible plan.
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No, it is not that bad. As I mentioned on this forum I had to give up of daily usage of SFOS based phone due to poor data safety. But I still interested in SFOS evolution so I didn’t reflash my XA2 back to Android and put spare SIM-card with that “terrible” plan in it.
Nevertheless I travelled with my Jolla without roaming plan (=I used WiFi only and swicthed cellural data off) and I never faced that issue. I’m sure I’d noticed that since roaming is quite expensive.
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I agree with kandelabra!
Mobile data off has to effect mobile data off, really off, really NO data connection, nothing, 0. It is an important thing, that the mobile data switch affects the real data connection deep in the whole system and not only on GUI or application level! This essential for data safety! Mobile data off HAS TO effect like pulling out the plug of the data connection!
Please fix this in next edition of SFOS!
And can someone please explain, what was the intention, not to do so from the beginning?
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Sure, that is to be expected. Thus me asking where it is enforced, with the implied question of whether there are alternatives to it.
Pointing out a possible better option (or just providing more details about the issue) could have better effect in attracting bug fixes than simply saying that things does not work as expected.
I did not mean to imply that it actually is done that way, i was merely asking.
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The reason for my excitement was 1st
on travelling in non EU countries there can arise immense costs for data roaming, if switching off the data communication for the home SIM card does not mean really off,
This can cause, that a prepaid card unexpected will be empty and then the user can neither use nor reload it if the user is “on the road” in other countries
and 2nd,
A reason to choose SFOS and not one of the two big A’s is also 1st for better privacy (and 2nd because it’s Linux and a mostly open software, for tinkering around in the system for fun and interest).
So I am a little bit disappointed if even with switched off data connection some data packets go around the world and nobody knows to whom. 
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