I and my family once had 7 monthly o2 contacts. When I upgraded o2 made an error (later accepted by their complaints review service - which by the way failed to even apologise!). This error resulted in continuous threats of court action and debt recovery agencies..... customer service was shocking to say the least and this took 6 months for o2 to admit fault and sole responsibility. Unsurprisingly, my family no longer have any business with O2..... However..... be advised that when you leave o2 they lock you out of your online account prior to the final bill being generated. Consequently, you have no idea how much is due to be refunded and the matter could be easily overlooked.... Be advised that the customer must APPLY in order to get a refund from o2. A deceitful policy and a procedure that clearly benefits o2 to the detriment of it's customers when accounts are closed. If you closed an o2 account you may want to check how much you are owed...... Appaulling customer service.