Will an AT&T GoPhone SIM work in an AT&T Motorola RZR?

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.

I think the phone is locked to a particular network. I suggest you can approach Mobileunlocksolutions.com and specify the country,locked network of Motorola Razr.Based on the availability you can get the permanent unlock service along with free instructions.