Hi you can purchase either iphone already locked with AT&T or else unlocked iphone else if you purchase iphone locked with some other network provider then you can't use AT&T sim in it until you unlock it ..You can easily unlock your iphone using sites like WickedUnlock.com here they are rendering remote unlocking service to unlock all model iphone from network provider .You first check that whether the service is available for your country and network provider ..If yes then pay for it and unlock your iphone then freely use it with any GSM sim ...