Americans now have to get permission to "unlock" their smartphone so it runs on more than one mobile network.
A 90-day time limit that made it legal to unlock phones without permission has now expired.
Many Americans unlocked their phones to avoid running up big bills when travelling outside the US.
An online petition has been started asking for unlocking without permission to be made permanently legal.
In October 2012, a change was made to the US Digital Millennium Copyright Act (DMCA) that temporarily allowed owners to unlock their smartphones without the need to ask their network beforehand.
Prior to the change, owners were typically charged a fee when they asked their operator to unlock a phone. Alternatively, users could buy unlocked versions of smartphones from manufacturers, but these handsets were typically more expensive than those locked to one network.
Users will now have to seek permission.
However, it is not clear what action will be taken against customers who ignore the law. The Electronic Frontier Foundation, which campaigns on digital issues, said in an email to Tech News Daily it should be up to the courts not the government to decide to what the DMCA applies.
In addition, many online services have sprung up that unlock phones for a small fee and some have said the change will have no effect on them.
Also, some US operators, such as Verizon, unlock all phones of a particular type they sell. AT&T is known to unlock all phones on an expired contract for its network.
So far, about 3,500 people have signed a petition on the White House website asking for unlocking to be legal all the time - 100,000 signatures are needed before the US government responds.
Unlocking a phone is distinct from a practice known as "jail-breaking" that opens up a phone so software from unofficial sources can be run on it. Jail-breaking remains legal in the US.