To transfer your phone number to giffgaff, you first need to ask your old mobile provider for a Porting Authorisation Code (PAC). You can use then use the transfer form to submit your PAC and choose a date for the transfer. If your number has not been transferred by the day after the scheduled date, you can contact an agent about it.
If your number still hasn't transferred by 9am on the following day, you can raise a query to a Member Service Agent. Make sure to complete accurately the following information: Existing giffgaff mobile number; new mobile number (the one you want to transfer), alternative contact number; PAC.
Please do not contact the agents before 9am on the day following the transfer date as the process would be still ongoing and they could not act on it.