I was having problems with my galaxy S4 with different suppliers gave me same unlock code again and again. As a result of my extensive internet search, I found this and this worked for me. I unlocked my phone with the same unlock code given by iGSMunlock.com. Hope this would be helpful.
Inset a NON AT&T sim and switch on the phone.
When it prompts you to enter the unlock code, dismiss the unlock code screen.
a. Go to the Dial pad and dial *#197328640#
b. Please click on number  UMTS >  Debugging screen >  Phone Control >  NETWORK LOCK
c. You will see 3 options
d. Please click on number  Perso Sha256 INFO, the screen will change slightly
e. Please hit the MENU button then when the menu pops up please carefully hit BACK to return to the previous screen
f. Next Please click on number  Perso Sha256 ON, the screen will change again to a one line message
g. Restart the phone on the screen by holding the power button and click RESTART
h. When the phone prompts for the unlock code simply enter the unlock code provided to you
i. Your phone is now permanently unlocked!