Sandy here from the giffgaff community team.
Thanks for posting on the community. Unfortunately, there will always be fraudsters and scammers out there who try to trick people into giving away personal information. I am so sorry that this has happened.
Please don’t click on any links in any email or text message you're not sure of. Genuine links from giffgaff will only ever begin with giff.ly or giffgaff.com. When in doubt, it is always better to just type giffgaff.com into your browser and access your account that way.
Smishing texts (like the one you have received) look like they have come from legitimate companies, such as giffgaff, with a link to a page that looks like the official website asking for ‘verification'. These pages ask for personal details that can include things like your username, password, full name, address, banking card information and security questions.
Please change the password on any accounts that you believe may have been compromised, for your giffgaff account you can do this here: https://www.giffgaff.com/profile/details#passwd (Please use a different, strong password for every account you access. A password manager such as 1Password or LastPass is also recommended)
If you have an active SIM, please forward the received text message to 7726. You will not be charged for reporting smishing texts to 7726.
I will be sending you a Private message which can be read from your profile page via your avatar. If you can respond to that message it would be superb.
Sandy & the giffgaff community team