Iphone says invalid sim how do i activate it?

So i just bought the iphone 4 today and put my o2 sim card in first it said no service but not it says invalid SIM what do i do to activate it?

Try bringing it back and asking them

You have to call your carrier to get the PUK code to unlock the phone so it will work on other carrier networks.

The phone is currently locked to a network. The only way to get this working with your device is to use an unlocking service like http://unlock-4.com/. You will need to know the original network that the device is on but this will unlock your device to be a factory unlocked device.

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