If you are the owner of the Samsung Galaxy S3 or the Samsung Galaxy Note 2, you will be happy to learn that you can unlock the device totally free of charge. Once it has been unlocked you can then use the device around the world, or switch carriers, providing of course you are not tied in.
Samsung Galaxy S3 & Note 2: how to unlock your handset
If you reside in the US and you purchased the device with AT&T, the Samsung Galaxy S3 will only work with that carrier. However if you unlock it you are able to use it on any network. You might even send the device to a relative that lives outside of the US and they could use it with any network they wanted. So here is how to unlock the Samsung Galaxy S3 or even the Note 2 in 5 easy steps.
Put a SIM card into the phone that is not from your carrier and switch the device off and back on again, with that card in the device.
Open your dial pad and put in *#197328640#. Head back to the Main Menu >  UMTS >  Debug Screen >  Phone Control >  Network Lock > Options Perso SHA256 OFF > and then wait for 30 seconds.
Wait for around 30 seconds then choose Menu and choose Back. If all is well you should see Network Lock.
Choose  NW Lock NV Data INITIALLIZ.
Just so that there is no mistake the sequence you need to type is: Asterisk Pound One Nine Seven Three Two Eight Six Four Zero Pound. You might call the asterisk the star key and the pound button might be called the hash. If all has gone well you should now have an unlocked Samsung Galaxy S3 or Samsung Galaxy Note 2. It seems that this code has been used by Samsung for quite some time and so it could very well work on different Samsung handsets.