Yeah if it's a T-mobile phone and T-mobile blacklisted the phone, it will NEVER work on T-mobile. Sim unlock might or might not work on another network such as AT&T as I've heard they now share blacklisting. Might work overseas.
Refund the money, get it unlock to see if it work on AT&T, resell it on CL/Fleabay and make sure the listing shows that it's a BL T-mobile phone.
*Samsung Galaxy S 4 M919 - Wicked ROM
*HTC One S - ViperOneS ROM
*Asus Eee Pad Transformer TF101 - KatKiss JB 4.3
us 4 16GB - The GN0M3 Project
uS One CyanogenMod 7 temasek's build. (Finally retired)