Vodafone prepay balance deducted for "ghost" data usage

I am a Vodafone prepay customer.

I have cell data turned off, but I am still being charged for data by Vodafone. I exclusively use wifi data.

I have turned off background data for all my apps.

I changed phones to confirm that it is not a device issue.

My phone shows that I have used 56 Kb of mobile data in the last month since I changed phones.

I have now installed an app NetGuard to restrict any apps from accessing mobile data.

But I am still getting charged for data usage.

The Vodafone chat reps give me instructions on switching off mobile data, which is already turned off?

I have an Android phone running Android 9. Location services are also turned off.

I am out of ideas now.

Has anyone else faced the same problem? Any suggestions are welcome.

Thanks