The Samsung Galaxy S3 and S4 remain popular even though newer devices have come out onto the market. Many chose to buy a locked handset due to the fact that they could get it cheaper with subsidies. However if they want to take the device to a different carrier they have to unlock the handset. Now there is good news as there is an unlock code going around allowing you to set your device free.
The leaked unlock code is said to unlock the Samsung Galaxy S3 or S4 so that you can move to the carrier of your choice.
To unlock your handset simply put a new SIM card into the device and switch it back on. Go to the dial pad and then enter *#197328640# and then follow the steps that are outlined below.
 UMTS >  Debug Screen >  Phone Control >  Network Lock > Options  Perso SHA256 OFF.
Wait for about 30 seconds and you can then head back into the main menu and you should see Network Unlock. You can then choose  NW Lock NV Data INITIALLIZ and you should now have a device that is unlocked.
You may wish to try using the unlock code with different handsets as it is said to work with the Samsung Galaxy Note 3 along with the Note 2 and other Samsung devices.