Apple iPhone

Hey guys, I was wondering if I could use my hong kong sim card on an unlocked iPhone. I have an iPhone that is already unlocked, and when I put my HK sim in it didn't work, it asked for some sort of network key, that I don't have or know how to get. I read about different programs and things like that, but my phone is unlocked already, so do I still need those? Can anyone help??

-iPhone drama