Unlocking your Samsung Galaxy S4 so you can use a different SIM card.We would share the method of unlocking your T-Mobile Galaxy S4 SGH-M919 and AT&T Galaxy S4 SGH-I337.
Make sure your device is backed up. Messing around with SIM carrier stuff can be risky.
1. Dial the Secret Number
Enter *#27663368378#,it will take you to the Service Mode menu. If the above code does not work for you, try this: *#0011#.
2.Now select  UMTS
3.When you get into the UMTS Main Menu,select  DEBUG SCREEN
4.At the DEBUG SCREEN, select  PHONE CONTROL.
5.In the Phone Control menu,, select  NETWORK LOCK.
6.In the Network Lock screen, you should see three options.select  PERSO SHA256 OFF.
7.Go back to the UMTS MAIN MENU, by tapping on the Menu button and then the Back key.
8.At the UMTS MAIN MENU, select  COMMON.
9.From the Common screen,select  NV REBUILD
10.At the NV REBUILD screen,Select  Restore Back-up.
11.Galaxy S4 will now reboot automatically.
Your Galaxy S4 will be unlocked.You’ll be able to use any other SIM card on it.