I Phone 3g 已解鎖但過唔到歌

I bought an IPhone 3G (美版) from 先達. The seller told me its a 已解鎖版. After using turbo sim i can use the phone feature, but i cannot download songs from itunes (version 8.0) to my phone. It says my sim card does not belong to their approved service provider ><

Is there any way to unlock it???? Pls help