Would it be possible to have two numbers associated with a SIM? it'd be useful when porting a number to giffgaff if the original number continued to work for a few days after the switchover.
If the user wanted to keep two numbers longer than this then it could be possible with the payment of a small monthly fee.
Of course if there's two numbers associated with one sim there'd have to be a way to decide what is sent as caller ID. I suggest the ported number should be the caller ID once activated, but if the user wants to keep both then they could specify the default number online when setting this up.