T-Mobile S III into an AT&T S III Unlocked with no code , No problems

Maybe its just me but all i hear or see is that you need to have certain codes to unlock  carriers or Sim unlock. With also paying a 30-50 dollar price never mind you.Mentioned before in my last post how i took a Verizon Samsung Galaxy S III put a AT&T Sim card and ha no problems what so ever no code no spending 30-40 dollars.. Well this friend of mine had a cracked glass of a T-Mobile Samsung Galaxy s # and thought to myself i have an extra AT&T Samsung Galaxy S III why not try this again. Except this time Make sure to back her contacts up with an app put it on her micro SD card set it on my AT&T samsung galaxy S III put the T_mobile Sim card in my AT&T Samsung  Galaxy S III Put the T-Mobile APN settings it actually recognized it and worked. Now I'm no genius by all means but all over YOU tube there telling you to have a code and spend money don't if you have problems maybe i can help . Well good luck and comment maybe i can help with an issue.