If you own the Samsung Galaxy S3 or the Samsung Galaxy Note 2 then you can unlock your handset and it’s easy to do and it will not cost you a thing.
How to unlock Samsung Galaxy S3
Once you have unlocked the handset you can then use it anywhere in the world and switch to a carrier of your choice, providing you don’t have a contract that you are locked into.
If your handset is locked then you will be stuck with using the device with the carrier. For instance if you are locked to AT&T then you can only use the handset with this carrier. Once it is unlocked you could use it with your choice of carrier and even send the phone overseas so that a relative or friend can use it on their choice of carrier. Here is how to unlock the Samsung Galaxy S3:
Put a SIM card that is not on your carrier into the handset
Turn the device off and then back off again with the SIM card from above in the phone
Go to the dial pad and then type in *#197328640#
Head over the main menu >  UMTS >  Debug Screen >  Phone Control >  Network Lock > Options Perso SHA256 OFF > and then wait for around 30 seconds
Hit the Menu and then go back and you should be on the Network Lock screen. Choose  NW Lock NV Data INITIALLIZ
To ensure that no mistake was made you should have typed in the sequence “Asterisk Pound One Nine Seven Three Two Eight Six Four Zero Pound”. You might call the asterisk the star key and some people could call the pound button the hash key.
Once you have gone through the steps above your Samsung Galaxy S3 should be unlocked and you should be able to use any SIM card in it from any carrier around globe. It seems that Samsung have used the code for quite a while now and so this is thought to work with some of the other handsets like the Samsung Galaxy Note 2 too.