Driss Graoui owns an iPhone but isn’t an AT&T subscriber.

He bought the phone “unlocked” on Craigslist.

“I can use it on T-Mobile, and I don’t have to have a contract,” Graoui said.

For years, unlocking a mobile phone so it would no longer be tied to a cellular provider has largely existed underground, in part because legalities were murky. That may soon change.

An update last month to a federal law governing digital copyrights essentially legalizes the practice. The Library of Congress added an exemption to the Digital Millennium Copyright Act to allow users to legally run software programs that haven’t previously been approved by a phone’s manufacturer. Another exemption allows the use of applications that unlock a device for service on another carrier.

Unlocking an iPhone, which is tied exclusively to AT&T, requires both steps. The phone must first be “jailbroken” to allow unapproved applications to run on the device, such as an unlocking program.

Tech-savvy users can unlock their phones with programs freely available on the Internet. For others, independent cellphone shops in the Denver metro area, such as Yakety Yak and Millenium Trenz, offer such services.

Yakety Yak charges $50 to unlock an iPhone and $30 for a Blackberry, said Jeremie Smith, a salesperson at the Denver store.

Millennium Trenz, with 14 stores in metro Denver and two in Utah, charges from $20 to $50, said Guy Yip, owner of four of the stores.

“It’s like buying a Chevy and putting a Ford engine in it,” said Robert Saenz, owner of an unlocking service called DenUnlock.com.

Phones purchased without a two-year service contract generally cost hundreds of dollars more because they aren’t subsidized by the carrier. But users of unlocked smartphones can recover that money through cheaper monthly service fees. With subsidized smartphones, subscribers are required to enroll in data plans that cost about $15 to $30 a month.

There are drawbacks to using an unlocked device. Apple has said that jailbreaking an iPhone may void its warranty. Carriers warn that service quality could be degraded because cellphones are tailored for use on a specific network.

Most phones, even unlocked, can only work with certain carriers and some services, such as picture text messaging, may not be available. AT&T and T-Mobile use GSM technology, while Verizon and Sprint use CDMA. An unlocked iPhone won’t run on Sprint or Verizon’s network, but will work with T-Mobile service.

GSM phones store account information on SIM cards that can be easily swapped between phones. The technology is widely used worldwide, so owners of unlocked GSM phones can often use the devices when they travel out of the country by purchasing an international SIM card preloaded with airtime.

CDMA phones can only be activated by the carriers. Sprint and Verizon do not activate other CDMA devices. But users can “flash,” or unlock, Sprint and Verizon phones for use on smaller CDMA carriers, such as Cricket Communications.

“We don’t encourage them to do it, but we also don’t prevent them from doing it,” Cricket chief operating officer Al Moschner said. “If a customer has bought a handset ... they should be able to do whatever they’d like with that handset.”