My sons phone broke, he isn't eligible for an upgrade and there was no insurance on it. I got my hands on a new AT&T ZTE Z431 but it's locked. I threw his sim card in there and the screen is saying "Invalid Card". Brief internet research is starting to prove that I can't unlock it without buying something and waiting for it to be delivered, which I can't really afford and don't have time for. Anyone know if there is a way around this?