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Not sure, you'll have to ask your provider if the pay as you go iphone you have is unlocked or not. And if not how much they will charge you to officially unlock it.
 
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I think you'll have to phone o2 and ask them to unlock it which my incur a charge.

I'd assume that an o2 pay as you go iPhone would be network locked.

http://shop.o2.co.uk/update/unlockmyiphone.html That may help.
 
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