Can't activate my iphone 5

I bought a iphone 5 in Australia a few days ago, when I came back to China and put my sim card in to activate it, it shows that the sim card is not valid. I had changed 3 cards but everything stays the same. I'm sure that this iphone is unlocked, and when I put the sim-card in it, it has the mobile signal and 3G service.
What's wrong with it?
THX.

Why are you "SURE" that your phone is unlocked?  It's acting VERY LOCKED.

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