I too have just had huge use reported these last 2 days. My phone (Samsung S21ultra) reports 600MB use (similar to my average) - but BT says 3000MB over last 2 days. Strange...
Have you reported it? I would be interested to hear what they are telling everyone who has this issue. I would think that it is affecting far more users than have either reported it to BT or who have commented on the Community.
Call BT up and ask what the data is being used for, my guess they'll say You Tube as that's what I've been told 3 times and virtually everyone on this thread has been told the same.
I'm inclined to agree. I've been told the 3 times its occurred that its You Tube, even though the last occasion on 1st/2nd August, my You Tube had been disabled 2 months before, so work that one out.
Just been told now that I have gone over my limit. I phoned BT 2 hours ago and also switched data off on my phone. Still managed to "use" data somehow according to BT. Huge (for me) data use since my Samsung S21ultra got a software update 2 days ago (over home wifi). Worrying...
Did they claim the usage was You Tube or something else, how did they explain that data usage continued to rise when you either had it off or turned it off when you received the SMS.
This morning I received a text message and an email to say that I have "now used all the data included in this month's plan allowance." When I check the usage online at my.bt.com it says that I've used about 3/4 of my plan allowance! Someone's telling fibs.