Maybe it's the high churn rate @persco
, people taking then dumping SIM cards for various reasons which makes the giffgaff numbers susceptible to this issue.
I really don't know, but when I am on my third number in what, 4 months because of it, (I've never actually given out the numbers I've had in that period, all calls and texts received have been based on the previous owners usage), in an attempt to stop it, there is clearly an issue which needs to be addressed.
If the numbers have also been signed up to the premium rate scams, some people could be in for an even bigger shock when they come to activate a SIM.
It is fairly easy to get around, like I said it's literally just a case of getting a PAYG SIM from another network and using that number, either directly or via porting, but that's not something which people should be having to do and it will put many off joining and staying.