From what I've read, Straightalk is basically a wholesaler of cellular service. They don't have their own network, they resell the service from ATT, Sprint, tmobile and Verizon. So it depends on where you live and what phone you buy as to what network you will connect to and whether you need a sim card or not. I want to use ATT and now they allow you to buy a sim for att to put in your unlocked iphone which is what I am thinking about doing. I think you can also buy a sim for tmobile.