Phone is using cellular data even when mobile data set to 'off'
I have a mobile data plan where I am only charged for days when I use data. When I don't use data, no charge. I have noticed, however, that I frequently get a charge when I have had the "mobile data" setting turned "off" all day.
It's an iPhone 6, with iOS 11.03.
First I noticed it happening on days when I did a software update. That happened several times.
Then I noticed it when I just restarted my phone. I was on a chat with Apple at the time, so I described it as it happened.
Today I noticed it when I took the SIM card out, then replaced it, but didn't restart the phone.
Each time, it has registered a tiny bit of mobile data usage, which my provider can see, and that causes them to charge me.
I've had two long support chats about this, but Apple haven't agreed it's a problem. They said they would escalate it - actually, the term was "tell someone", so I think I'm being ignored.
I appreciate that only people using this kind of phone plan would notice it, and it's an unusual plan.
So I'm just wondering if anyone else has noticed such a thing?
iPhone 6, iOS 9.2.1