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To transfer your existing number to giffgaff from another service provider
You first need to get a PAC code from your existing provider.
Now input it on the "Transfer your number to giffgaff" page (follow the link on your my giffgaff page, in the handy links box, bottom left on the main site).They will set a date for the transfer.
Any active goodybag or existing credit will remain after the transfer, though you will lose service for up to a day during the transfer. You do not need to reactivate after the transfer.
sim swap: please see here:http://community.giffgaff.com/t5/Learn-giffgaff-Top-Tips/How-to-use-SIM-Swap/td-p/6885194
porting your number: http://giffgaff.com/support/transfer
Do you mean your number from a different network provider?If so,contact your previous network and ask for your PAC.You should receive this within 2 working hours.Once you have received your PAC and your giffgaff sim has fully activated,fill in the transfer your number form found in the handy links section on your homepage.