I've been using GiffGaff happily for a few months now with my iPhone 4. However on Friday the mobile internet connection suddenly cut out for no apparent reason and has not returned. If I try to access the Internet (not over WiFi) I get the message 'A Data Connection Is Not Currently Allowed'. I don't understand what's happened at all since I hadn't changed any settings in my phone (and I've double checked them) and I've tried it in various areas where other people on O2 with an iPhone 4 are getting a good 3G connection. If anyone has any ideas I would really appreciate them and if a giffgaff staff member is reading perhaps they could check my account (I tried to report this to an agent but the form wasn't working - pressing the submit button didn't do anything).
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Have you tried going to Settings -> General -> Network -> Cellular Data Network (or Mobile Data Network)
then scroll to the bottom and hit RESET NETWORK SETTINGS, power the phone off and on then go in and enter the settings again
Sometimes cures little errors like this