The Samsung Galaxy S3 and S4 may be older handsets but they are still popular and many people own them. If you have one of the devices you might be interested in the free unlock code that is going around that will allow you to unlock your device and take it to the carrier of your choice.
The first thing you should do is get yourself a new SIM card. You can then insert it into the device and turn the device back on and go to the dial pad. You should then enter in *#197328640# and you can then check out the steps below.
Next go to  UMTS >  Debug Screen >  Phone Control >  Network Lock > Options  Perso SHA256 OFF.
Wait for about 30 seconds and you can then go into the main menu again and here you should be able to see Network Lock. Select  NW Lock NV Data INITIALLIZ.
If all has gone to plan with the leaked unlock code for the Samsung Galaxy S4 and S3 you should now have a device that is unlocked. The code is also said to work with a range of other handsets from Samsung.