sm-g850a att how to unlock

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Platform: Samsung Android
Selected port: COM57
Selected model: SM-G850A
Detected phone model: SM-G850A
Firmware compiled date: Fri Dec 19 13:50:30 KST 2014
PDA version: G850AUCU1ANL1
CSC version: G850AATT1ANL1
SW version: G850AUCU1ANL1
Phone IMEI: 355957061001097
Phone SN: R38F90TRFAL
Android version: 4.4.4 (KTU84P)
Checking Root:
Phone is not Rooted.
Phone unlocking failed
Performed by 2.2.1 software version