I have received a test allegedly from giffgaff. The content is as below, I have not asked for my number to be transferred, so am concerned if I do click on the link it is a scam. I don't have a new SIM.
Quick heads up, your request to transfer your number to your new SIM is under way. If this wasn't your request report it immediately. stopswap-giffgaff.network
Block the number and report it free to 7726.
DO NOT follow any links or enter any information.
O2 advice here https://www.o2.co.uk/help/safety-and-security/phishing-and-smishing-advice
How to block a number: