Unhappy [Q] Sony Xperia Ion LT28at Won't SIM-unlock!

Bought an LT28at off eBay, locked to AT&T. Checked and was surprised to see that it only had two SIM-unlock attempts left.

Bought an unlock code (YES, I sent them the correct IMEI) and entered the unlock code. Took my time, it took two minutes to enter the entire code. Failed to unlock.

Long story short, the reason why it has only two unlock attempts left is previous owner tried to unlock (emailed me the exact same code I have) and it would not work for him either.

Correct IMEII, correct code, entered correctly. Triple- and quadruple checked.

SO... I have the correct unlock code for the phone, and both the previous owner and I could not unlock this phone. So it's not human error. What next? Would rooting help? Any suggestions?

Thanks!
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