Well i'm unable to manually shut off my iPhone (top button is broken) so the sales guy put the new SIM in and told me to let the phone die, then charge it and it would pick up the network when it turned back on.
Wait! You bought your phone even though you saw the top button is broken?
I wouldn't take a phone in such a condition. If you still have problem tomorrow, you cam go back to the store and ask to exchange to a new one.
Just went back to best buy and they said the same thing, that my old provider probably hasn't unlocked it yet (even though they said it would be fine after 1pm today) so i'm just waiting till tomorrow at this point