up_the_saddlers - turning off data is not an acceptable course of action for people who reply on it. "two wrongs don't make a right".
As I've already said on Twitter I am happy that GG is chasing things up themselves, however the fact that this has happened at all is totally unacceptable. Whichever company/individuals (not likely) re responsible should be hung out to dry (also not likely, but it's what should happen)
This can only be described as a monumental **bleep** UP by O2. It needs to be dealt with, and quickly.
I have checked and can confirm that my mobile number is in the x-up-calling-line-id header when using 3g. I know that giffgaff are not ultimately responsible for this problem but it is they that need to be REALLY on the case of O2 to get it sorted quickly.
I suggest that all giffgaff users disable their 3G wireless access until this is fixed. This is a spammers dream!