I'm a long time lurker and benefited greatly from the postings here at xda.
So I'd like to contribute something back.
After trying many methods and reading all the posts in the various S4 forums, I finally found a way for SIM unlock that works for me.
I have tested it, and it works on my T-Mobile S4.
Credit to these posts:
- KaneHusky's post for the dialer code
- This thread on how to unlock
- chavxxx's post pointing to the right final step
- Is this for real? Does it really unlock?
YES! Many people have tried it, and most people have successes.
If you are having problems with the unlock process, please read the rest of the FAQ.
- Will this SIM unlock work on my model of Galaxy S4 that is not T-Mobile (SGH-M919)?
This unlock has been tried on the T-Mobile (USA) version of the phone, if you're coming from any other model, YMMV, it might or might not work.
That being said, many others have tried it on the AT&T model, and on the international I9505 with success.
- Do I need to be on stock rom?
Yes, you need to be on stock rom for this to work.
One of the main causes of failure to unlock is people not flashing back to stock rom first. The reason is the special dial codes only work on stock roms.
- Is the unlock permanent? Do the unlock last through reboot/reflash/etc.?
Yes, the unlock is permanent.
You can reflash a custom rom, or do anything you want to the phone, and the unlock will stay.
- In step 7, my phone is already "SHA256_ENABLED_FLAG", what should I do?
Great, this is the setting you want.
It means your phone already allows you to make some settings changes, it DOES NOT mean your phone is already unlocked.
You should continue with the rest of the process until the last step.
- In step 14, backup missing.
If the golden backup doesn't exist, try the work around found by GrndSers-Rcng:
- How do I know the unlock is successful?
If your phone is not unlocked, when you put in an unauthorized SIM card, a dialog pops up on your phone asking you to enter the unlock code.
If you're successful with this procedure, there would be no popup dialog asking for the unlock code when you put in any SIM card.
Here are the steps:
- Goto phone dailer, and enter *#27663368378# for the debug service menu.
- In "ServiceMode", select  UMTS
- In "UMTS MAIN MENU", select  DEBUG SCREEN
- In "DEBUG SCREEN", select  PHONE CONTROL
- In "PHONE CONTROL", select  NETWORK LOCK
- In "NETWORK LOCK", select  PERSO SHA256 OFF
It should show:
SHA256_OFF => SHA256_ON
- Now touch the first line SHA256_ENABLED_FLAG
It should show:
MENU NOT EXIST
PRESS BACK KEY
Current Command is 116631
- Now press the menu button and select "Back"
It should show:
SHA256_OFF => Not Change
- Again, press the menu button and select "Back", and you're back in the "NETWORK LOCK" screen
- Press the menu button and select "Back", and you're in the "PHONE CONTROL" screen
- Press the menu button and select "Back", and you're in the "DEBUG SCREEN" screen
- Finally, press the menu button and select "Back", and you're back in the "UMTS MAIN MENU" screen
- Select  COMMON
- In "COMMON" screen, select  NV REBUILD
Now you are in the "NV REBUILD" screen, you should see the following text:
You can Restore Cal/NV
- Now for the final step.
In "NV REBUILD" screen, select  Restore Back-up
The button lights for menu and back should come on, and the phone will auto reboot in a few seconds.
Note: chavxxx's post of selecting " Load default NV" DID NOT work for me.
Congratulations, your phone is now unlocked.
Stick in a sim card from a different carrier and try it out.
Please post followups of your experience with this unlock method.
Thanks, and good luck!