Sim card failed in samsung solstice?

[T A]

So I live in the states, and am staying in brazil for a few months. I bought a prepayed sim for my cell phone. The phone is already unlocked and it should work fine since its an at&t world phone. However, when I put in the pre-payed Sim from Claro into the phone it says "Sim card failed" and the only two options are "SOS" and call put you cant call anyone except emergency calls. The Phone is a Samsung Solstice if that helps any help would be appreciated. I already talked to Claro, and they said that they cant do anything about it