In a nutshell, the way round it is to go into Settings - Network Settings - Preferred Network - then change from 4G to 3G only or 2G/3G. On days you want to use 4G eg. actual call, or internet when out and about rather than just using your home broadband for internet, switch back to 4G for the day.
Like plenty of others, I had this issue myself with a Nokia smartphone using Android 10, which I posted here, (but it wasn't happening with an old 3G PAYG!). I then played around with various options, mobile data, location off, etc. The above was the solution that I found to work. What's happening is that even with Mobile Data switched off, the phone is checking in at the start of every day with the nearest 4G transmitter (just a 4G big brother thing); this is a miniscule amount of data but gets picked up and gets you charged.
What annoys me though is that this is non-chargeable data; even if you have no balance left, the daily check-in will still be made. It's like making only a single call in a day to check your balance 1345; the call won't be charged for, no £1 will be taken . This is where VF should act, if the data is non-chargeable, no charge should be made.
So that's my diagnosis and workaround. Hopefully works for you others with a similar issue.
