I have always found O2 customer services to be excellent. I haven't had the need to call them much but found them great when I had my wallet and iPhone 4s stollen whilst in Spain. They instantly blocked my sim and even put me through to Barclays to stop my visa card. When I got I had a spare micro sim and they instantly unblocked my number and put it on the microsim. The calls were answered promptly and by someone in the UK, nice broad Yorkshire accent.