Just dial the following keys *#197328640#
 Debug Screen
 Phone Control
 Network Lock
Options Perso SHA256 OFF
(after choosing this option, wait about 30 seconds, then go back one step by pressing the Menu button then select Back, now you are in
 Network Lock then choose
 NW Lock NV Data INITIALLIZ ..... wait for a minute then reboot your phone
Don't pay high unlocking fee's on eBay! Say no to them with a big smile!!
None of this is my original doing or my intuition.. I have no idea how this guy figured this out, however I have done this and can verify that it did indeed work on my AT&T Galaxy S3... Model # = sgh-i747.. I was not able to comment my progress on the original thread because I am still under the 10 posts.. So I am going to keep reposting methods that have tested tried and trued.. I put a tmobile sim in my device and it registered at 4g speeds.. Did not commit a test, however i just slapped my red pocket sim in and edited the apns and it was ready to go.. I believe this would be the case for any of the subsidiaries.. So big thumbs up, thanks, and props to the OP.. Here is his link
His name is DaveenJay.. Im not sure where he found it, but thats where I started.