iOS 26.1 using mobile data when it should default to wifi - costing me money

ios update 26.1 using celluar/mobile data although the main switch is turned off

I always have mobile data turned off and only turn it on when I need to use it to send a what's app. After the latest ios update 26.1 I shockingly discovered that my facetime calls didn't use wifi but mobile data and ate up my call credits. I also see data charges when my phone was sitting at home so it must have done some larger background download using mobile data again and I can't see what it did. This update is so bad it's actually costing me money. I meanwhile saw that the default calling functionality was set to mobile data? Why? And that setting seems to have overruled/overridden the main mobile data switch. I even turned off mobile data for the phone app and it still used mobile data when using facetime. I did think from a design perspective saying "mobile phone" instead of facetime call was weird and confusing and I didn't trust it. Well, how right I was.

I just read in the mobile data tab how it works and it says the mobile data on/off button determines if mobile data is used at all. Well, it didn't. So I would say this is a bug. Please fix this asap.

The design choices are also not the best in this update. They became unclearer and apple is supposed to be design king. I wished I hadn't done this update.