Check the O2 service in your area.
If all good,
Go into your Phone's Settings and find the Network section.Do a Manual scan for Networks.When the list populates
pick any Network other than GiffGaff or O2.When it fails then Pick Giffgaff and shut the phone down for a few minutes
and then restart it.
Also if you can connect to the Internet with the Phone install the free GiffGaff app,
And then shut the phone down for 5 minsThis should install the correct GiffGaff Network settings better than
the Sim normally does and if you have to go find and use the Reset Network Settings on the phone.On many Android phones this is in the Network settings or in the Back up and Restore section but only choose this option as you don't want to do a,"Factory Restore!