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Try carrying out a manual roam by following this guide: http://community.giffgaff.com/t5/Tips-Guides/Manual-Roam/m-p/3345471#M32502
1. Go to settings
2.Go to carriers
3. Switch off automatic and manually change the carrier to something other then O2-UK i.e vodafone or orange
4. Come out of settings and wait until it fails to connect to the other carrier
5. Once it fails repeat the process and this time choose O2-UK
This is an internet-only service, there is no call centre as such.
With regard to your issue, begin by checking the giffgaff network status in your area to see if there are any reported problems (mast maintenance etc.).
If the signal is fine, next step is to try manual roaming by following the link to the guide here:
That can help sort out various types of issues!