Do they really? Their website tells you both arrears or in advance depending where you look.
Go to your current bill page (which, annoyingly tells you nothing about the billing period) and scroll down to the graph of previous bills. To the right of this graph is a series of tiles for previous bills. Click on anything other than the "latest" bill, and it will open - and there it says you are being billed in arrears (ie, for the month just passed).
However, go back to the graph and there's a link below it that says "view and print older bills". If you look at previous bills there, it will tell you you're being billed in advance (for the month yet to come).
I've spoken to O2 customer service about this, and they insist that they've ALWAYS billed in advance and as a result, at the end of a contract they will pay a pro-rata amount back to me for the last advance payment.
They also said EVERY mobile company does this, but for the life of me I cannot remember EVER having received a refund at the end of a contract (and I've used just about every provider in the last fifteen years)
