Q: how to unlock old iPhone from now-defunct CINGULAR carrier

I have this old iPhone, maybe 3G or perhaps 3GS. The old sim card was from Cingular. I'd like to unlock it and just use it as a back-up phone.

I've erased it and reset the settings but when I try to connect with iTunes, iTunes tells me to insert proper carrier (i.e. Cingular (ATT?))

Any advice would be greatly appreciated. Thanks.