This seems to be a regular complaint now. O2 needs to sort this out and the only way to get anything done is for all customers affected to make a complaint. I think no matter what you try won't fix this, it's an o2 system problem. @MI5 himself had the same problem and he will have tried all the checks he has mentioned before having to change network.
If you don't have any luck sorting this out then make the complaint.
Hey @Anonymous welcome to our forum! Sorry to hear you've had unexpected increase in your data use. Did you check in case it might be the wi-fi assist mentioned above? Did you have it enabled by any chance?
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