Iphone 4s won't activate

My brother bought a used iphone 4s, he put a o2 sim card in it and it has full bar and 3g signal but no carrier name on it… He tried activating the phone but it says:

The SIM card inserted in this iPhone does not appear to be supported.
Only compatible SIM cards from a supported Carrier may be used to activate iPhone. Please insert the SIM card that came with your iPhone or visit a supported Carrier's store to receive a replacement SIM card.

It says this on the phone and if you try it through iTunes. Does he need a replacement phone and if so would apple charge him or would it be a free replacement?

What happened is you didn't do you research.iPhone's are usually bought as an upgrade/new line and cost roughly $200. The reason its only that much is because the phone has been locked down to that carrier and your now stuck on a two year contract. The used phone you bought has that same problem.

In order to fix it you need to on e of the following:
1) go to the original retailer and demand they unlock the phones SIM
2) go to a third party unlock my SIM card web site
3) jailbreak the phone and unlock the baseband for the sim yourself

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