SGH-T999L Phone is successfully unlocked!
 

Welcome to Octoplus/Octopus Box Samsung software version 1.9.2
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To connect SGH-T999L phone you have to perform the following steps:
1. Disconnect USB cable from phone.
2. Go to Menu->Settings->More->About Device.
3.Tap on "Build number" 7 times to enable developer options.
4. Go to Menu->Settings->More->Developer options.
5. Turn on "USB Debugging" option.
6. Go to dial window.
7. Enter "*#0808#" number.
8. Select "DM+Modem+ADB".
9. Press Home button.
10. Go to dial window.
11. Enter "*#7284#" number.
12. Set "UART" to "PDA" value.
13. Restart phone.
14. Press "Read Info" in software.
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Platform: Samsung Android
Selected port: COM19
Selected model: SGH-T999L
Mode: Normal
Phone model: SGH-T999
Firmware compiled date: Nov 01 2013
Firmware compiled time: 03:00:00
Firmware released date: Mar 13 2014
Firmware released time: 12:53:45
Phone IMEI: 355880-05-34XXXX-X
SW version: M8960A-AAAANAZM-1.0.443097
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Platform: Samsung Android
Selected port: COM19
Selected model: SGH-T999
Mode: Normal
Phone model: SGH-T999
Firmware compiled date: Nov 01 2013
Firmware compiled time: 03:00:00
Firmware released date: Mar 13 2014
Firmware released time: 12:53:45
Phone IMEI: 355880-05-34XXXX-X
SW version: M8960A-AAAANAZM-1.0.443097
Unlocking phone...
Creating NVM backup
Reading NVM done!
ADB server killed.
Please, press "OK" button on the phone to allow USB debugging..
Firmware compiled date: Thu Mar 13 12:18:14 KST 2014
PDA version: T999LUVUBNC1
CSC version: T999LTMBBNC1
SW version: T999LUVUBNC1
Phone IMEI: 3558800534XXXXX
Phone SN: R21D79S0KZM
Android version: 4.3 (JSS15J)
Cheking Root
Phone is Rooted
Reading security backup, please wait...
Backup saved as C:\Program Files (x86)\Octoplus\Octoplus_Samsung\BACKUP\Security-SGH-T999-3558800534XXXXX-21-04-2015_16-01-06-.asec file
Deactivating MSL...
MSL deactivate successfully
Phone will restart now. Please don't disconnect cable!
Firmware compiled date: Nov 01 2013
Firmware compiled time: 03:00:00
Firmware released date: Mar 13 2014
Firmware released time: 12:53:45
Incorrect IMEI
SW version: M8960A-AAAANAZM-1.0.443097
Found phone on COM19
Repairing NVM for SGH-T999 phone...
Phone is successfully unlocked
To finish the operation, please restart the phone via phone's menu (don't disconnect the battery!)

10x again for your good report. :)