I've been with O2 for over 15 years. Never again. I'm very disappointed of customer service. In a short story.. I tried to moved from pay as go to pay monthly on 10th. Next day I brought everything back to the shop. There was no problem with refund. Because I used to 'pay as go' and contract was 'pay monthly' , I've got a new sim card. My mobile number shouldn't change at all. After refund my phone number disappeared! O2 wasted my time and money. I've been on a phone for hours every day. I had to visit the shop nearly every day. Every day I've been told to wait 24 hours. I've got 5 diff sim cards that supposed to be my phone numer but it none of them worked. My bills, bank, all important things are connected to my mobile number and O2 was not helpful at all. I know that someone made mistake on a computer system. Finally after 12 days I've got my phone number activated.