From the main screen of your iPhone, tap Settings > General > Network
Tap ‘Cellular Data Network’.
Under ‘Cellular Data’, tap APN and change ‘idata.o2.co.uk’ to ‘mobile.o2.co.uk’ (‘o2’ needs to be written with a letter ‘o’, not a zero). You don’t need to change your username and password.
Ignore the APN box under ‘Visual Voicemail’ – it’s not available if you don’t have an iPhone Pay Monthly tariff.
Under MMS, tap APN and change ‘idata.o2.co.uk’ to ‘wap.o2.co.uk’ (‘o2’ needs to be written with a letter ‘o’, not a zero). You don’t need to change your username and password.
Turn your iPhone off and on again.
http://shop.o2.co.uk/update/settings.html