My partner got an SMS from "Giffgaff".
No phone number the text originated from, just the word "Giffgaff" as the sender.
She's with BT Mobile (EE).
The message stated something like "Your Giffgaff number is about to expire, please login to keep your number active. Please visit
http://giffgaff.something.com/ to keep your number."
I visited said site and it was an exact clone of the giffgaff site.
Entering a dummy username/password it told me I successfully logged in but now needed to confirm my card number that she last used to top up.
Pretty convincing scam for some Giffgaff customers (I'm one) although I noticed the backwards url immediately when my partner asked why Giffgaff were emailing her.