If you currently have an AT&T account, then all you need to do is put your SIM card in it and it will work because the Iphone is branded under the AT&T network. If you have another GSM network like T-Mobile, then you will need to buy an "unlocked" Iphone that has been unlocked to work on any GSM network. If you have a digital network (Sprint, Nextel, Verizon), then you can't use the Iphone.