@jt_21. @Cheers for that things must have changed over the years
I think they have Germansheppard234 but the info still states
"Yes. giffgaff accepts O2 vouchers, these come in the following denominations: £10, £15, £20, £25, £30, £35 and £50.
Please note you won't be able to call the top-up number printed on the voucher. Instead, you will need to call the giffgaff top-up number: 43430, or head online to redeem your voucher"
BUT you CAN call 4444 , I already mentioned this in 'Contribute',... ages ago ... the wording still has not been corrected.