OK, let's firstly eliminate 'possibilities'.
Going through this list step-by-step will isolate the problem to either your phone or to the network.
Feel free to skip any parts that you have already tried, e.g. 'was your SIM working previously?'
Sorry, it is a bit of a 'list', it is a 'catch all possibilities' reply but it will certainly help you to pinpoint the problem.
Firstly, if you have just activated your SIM it can take up to 24 hours for the full activation process to complete, a periodic restart will help.
Have you restarted your device since the problem became evident? This simple action often clears 'problems'.
If you have an iPhone, please try a resync with iTunes.
Do you have an active goodybag? Please dial *100*7# to check
Do you have credit? (needed if you don't have a goodybag or for any international calls/texts) Please dial *100# to check
LOCAL SERVICE PROBLEMS
Are there local service issues? Please enter your FULL postcode here to check https://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
Your current number and any goodybag / credit will transfer as part of the process.
Don't forget to back up any contacts on your SIM as these do not transfer as part of the process.
I hope that this helps you to solve the problem.