Make sure you have credit or goody bag, Dial *100# shows cash credit Dial *100*7# shows goody bag minutes Also remove sim, clean and check for damage. Turn airplane mode on then off.
"IF USING INTERNET check mobile DATA is set to ON in phone settings."
APN+mms settings. Please use these correct settings. All phones. Try updating phone to latest software first, usually loads giffgaff APN. APN and MMS are in this link. The giffgaff app on your phone will give the correct settings for that phone and you input manually. http://giff.ly/gg_settings
NETWORK RESET. Go to settings,more, mobile networks, network operators, choose O2 or giffgaff and phone will lock onto the signal. Job done.
MANUAL ROAM In stubborn cases please do this, it can make a difference. You can do a manual roam by first selecting a non O2/giffgaff network, the phone will set but not provide service, go back to network list and select O2/giffgaff again. Also remove sim, clean and check for damage.