If you’re experiencing an issue with signal, it can usually be narrowed down to a phone, SIM or local mast problem. Please try the following and report back:
Reboot your device
Network / mast issue
Try to verify with other local O2 / giffgaff users around you if they are experiencing similar problems.
Test your giffgaff SIM in another unlocked phone. If the same SIM exhibits similar issues in several devices, then it may be faulty. If you do not have a spare unactivated SIM, then order a new SIM here. You can then do a ‘SIM swap’ to transfer your existing phone number / goodybag / credit to this new SIM.
Test different known working SIMs in your phone. If all SIMs show signal issues in the same device, then there could be a potential problem with your handset.