I just bought a AT&T RAZR from my friend's soon-to-be stepfather for $50. However, when I put in my GoPhone SIM, it just says "Unregistered SIM", then tells me to "Check SIM". When I had my old RAZR, it offered a prompt for a subsidy code, and I had a local cell phone company SIM. It's not doing it for my GoPhone SIM. Why? Is there anyway to fix it?

I really like the look and feel of the RAZR and I want it back.