UK O2 Iphone 3G "Could not activate cellular data network. You are not subscribed to a cellular data service."

If your Iphone 3G stops accessing the web and you get the message "Could not activate cellular data network. You are not subscribed to a cellular data service.", in a PAYNGO O2 3G Iphone even if you've the monthly payment in order, then you are facing the same problem I did. I went and did some experiences with another SIM and somehow the network settings for the APN changed from payandgo.o2.co.uk to mobile.o2.co.uk. Just revert the unwanted change under "Settings -> General - Network - Cellular Data Network", change the APN back to payandgo.o2.co.uk (username: vertigo , password: password) and you'll be surfing again. :)