Which company do you use? A sim card is a little card that is attached near the battery of the cell phone. I know that tmobile uses them and I think cingular does also. So if you have one of those than what you want to do should work fine. But if you have verizon which doesnt use sim cards then I dont know the answer to your question.
Take the battery cover off your phone and take the battery out. If there is a small rectangular chip underneath the battery that you can slide in and out, that is your sim card. If you dont have a sim card, then you cant use an unlocked phone. I would say that Alltell probably doesn't use sim cards but I have no idea. Check it out.
that means your phone is still locked, you need to call the company that has its brand on that phone and ask them to unlock it.. they will email you a code within a few days.. if you enter the wrong code multiple times the phone is permanently screwed