Why won't my GoPhone simcard work with iPhone 3GS?

Okay so, i had an HTC freestyle, got the AT&T GoPhone plan with it but my HTC got messed up completely so i got the iPhone 3GS and put in the AT&T sim card but it says "no service" and yes, i paid my bill. Anyway, i called AT&T and they told me i have to go to the AT&T store and get a new sim card and have them unlock it but with a phone that is not the iPhone 3GS, or something like that. How does this work? I've seen other people put their GoPhone Sim cards into their iPhone 3Gs, i don't know why mine is acting different. I just want to know if this ever happened to anyone else and how will it work? I'm going to the AT&T store but i just want some info first. Help? Please? I'm a very impatient person ]: The iphone is unlocked.meaning it can work on any phone company.also, its at&t anyway.

Because you don't know what you're doing.

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