Settings need to be done first
APN+mms settings. Please use these correct settings. All phones.
Try updating phone to latest software first, usually loads giffgaff APN. Or put settings in manually.
All your settings for APN and MMS are in this link. Go slowly and check all you do is exactly as shown.
reboot after with a 5 minute wait while phone is off.
The giffgaff app on your phone will give the correct settings for that phone and you input manually.http://giff.ly/gg_settings
Now do this and include part two
NETWORK RESET. Sometimes network setting can drift and a forced reset can help locate a better signal. Go to settings,more, mobile networks, network operators, phone will now search networks and give a list please choose O2 or giffgaff and phone will lock onto the signal. Job done.
Note:: It's at this moment if your signal problem was severe you can do a double reset by first selecting from the list a non O2/giffgaff network, the phone will set onto that network but not provide service, go back to network list and select O2/giffgaff again (doing this can clear old O2details that may corrupt a new O2 choice, doesn't happen often but can be useful)
SOFT REBOOT. Try a soft reboot to force phone to reset to giffgaff. Hold power button down until options show, choose power off.Wait five minutes while phone is off. This let's decay complete. Then hold power button down until phone wakes up, it will then restart and reset itself to giffgaff