But if you already have an AT&T Galaxy S4 and you aren’t locked into a contract, you can easily use that phone on other’s network. The process is simple, but requires a little setup, and for some, sacrifice. Thanks XDA Forum Member sush143555 shared a method to SIM-Unlock the AT&T Galaxy S 4.
- Launch the Phone app on your Galaxy S4, and dial in: *#0011#
- Press the Menu button followed by the Back key. Then, hit the Menu button again and select KEY INPUT. Enter 1 and press the OK button to enter ServiceMode.
- Press Menu one more time and tap back again. This will pull up the Main Menu for ServiceMode. Tap  UMTS.
- Now select DEBUG SCREEN, followed by  PHONE CONTROL.
- From the next screen, pick  NETWORK LOCK.
- You’ll get three options. Hit the one that says  PERSO SHA256 OFF.
- Now, you need to get back to Main Menu. To do this, hit the Menu button followed by the Back key. In Main Menu, now pick COMMON, and then  NV REBUILD.
- The phone will tell you that a Golden Backup exists. Tap  Restore Back-up.
Your device will now go through a series of “conditions”, where it will first freeze, then get on a black screen with Blue LED on and both capacitive buttons on, after which it will reboot automatically. The whole cycle might take a couple of minutes, so be patient. Once it powers back on, it’s network unlocked, permanently.