How do I use my Cingular/AT&T sim card with a GoPhone?

To prevent me from closing my account, an AT&T representative told me I can buy a GoPhone and use it with my regular SIM card. I bought the phone and it recognizes my numbers on the SIM, but it won't send or receive any calls.

Does anyone know how to get the phone to work now, since putting the SIM in the new phone didn't work? Is there something special I should have done with the phone? Do I have to activate the GoPhone SIM first?

**Just to add, my SIM still reads Cingular. The old phone was a Treo and the GoPhone is a Nokia C3.

Thanks!