Giffgaff run on the o2 network and you can either use o2 locked phones or any unlocked phone
If your phone is locked to another network, then use Unlockipedia to get it unlocked.

giffgaff unlockapedia will help you get your phone unlocked

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Just click the image on the left

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To order a FREE giffgaff sim with £5 FREE credit on it Go to bit.ly/Free5Pound or bit.ly/5poundsim

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Click this banner
Click on the banner on the left to order a sim card with £5 FREE credit on it