I had never given much thought to unlocking a cell phone for use internationally as I use a pre-paid Nokia (T-Mobile) phone that costs me about $100 per year. I entered the unlocking code a few years ago and found that it was already unlocked and should have worked in Thailand, but it didn't.
It appears that unlocking some cell phones against the wishes of the company is illegal in the USA due to a copyright law.
http://www.cnn.com/2013/03/25/opinion/khanna-unlocking-cell-phone/index.html?hpt=hp_c3
More on the Digital Millennium Copyright Act of 1998 here; http://en.wikipedia.org/wiki/Digital_Millennium_Copyright_Act_of_1998
Ranb
It appears that unlocking some cell phones against the wishes of the company is illegal in the USA due to a copyright law.
http://www.cnn.com/2013/03/25/opinion/khanna-unlocking-cell-phone/index.html?hpt=hp_c3
(CNN) -- On January 26, a ruling by the Librarian of Congress made unlocking a cell phone for use on other carriers illegal under the Digital Millennium Copyright Act of 1998.
Unbelievably, such a policy means that those unlocking their phones could face up to five years in jail and a $500,000 fine. This ruling affects average people, international travelers and hundreds of thousands of our service members.
It's unfortunate that AT&T and Verizon's main lobbying organization was the main proponent of this ban (the Wireless Association).
More on the Digital Millennium Copyright Act of 1998 here; http://en.wikipedia.org/wiki/Digital_Millennium_Copyright_Act_of_1998
Ranb