It is now illegal and against the US law to unlock your cell phone without carrier’s permission.
The U.S. Copyright Office and Library of Congress are no more allowing to unlock phone, it has just become illegal as part of the Digital Millennium Copyright Act (DMCA).
When you buy a “carrier locked” smart phone/cell phone, it’s usable by only one carrier. That refers just to the carrier from whom you bought the cell phone service. You must obtain permission from your carrier to unlock the phone or face a potential fine of up to $2,500. Anyone found unlocking handsets for profit could be forced to pay up to $500,000 and may even go to jail, according to ABC News.
Some people purchase various types of software online or make changes, edits and tweaks to the software in the phone to unlock cell phones. The changing of the software in the phone is now illegal. You can’t unlock your cell phone until you get the permission in written so that you can prove you have permission to unlock your cell phone, getting the carrier’s permission in writing is very difficult due to the new law and also the copyright. You can refer to the Digital Millennium Copyright Act (DMCA). You can still legally purchase an unlocked phone new (though usually for a higher price).
The law is in effect from January 26, 2013 and you need to obey it.
You can read the full docket here