I had a mobile payg which I kept for emergencies.
Most of the time it was switched off and used occasionally for the odd call and to access the weather.
Recently I received a text from o2 saying "you only pay a £1 for using data today" which I ignored.
The following day I again received the same text and again ignored it.
The next day same again.
I never thought anything of it until I noticed that my balance was disappearing.
£3 had just disappeared despite not using my phone at all.
I then tried to contact customer services and eventually got through.
The person I spoke to, in response to my appeals for a complaints address, cut me off.
A few days later I find all of my personal data in the phone has disappeared completely.
No refund and no apology.
So, I would say to any potential customers of 02 don't trust them even for a minute.