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Nokia, the world's largest maker of cellphones, has been running ads that read “Open to Anything” and “Unlock your Potential.” The company wants cellphone buyers to know that its phones can be used with whatever carrier they choose, unlike a certain other phone that has been getting considerably more attention lately: Apple's iPhone. That phone is locked for use with only one carrier, the one chosen by Apple.

Nokia says its advertising campaign is not targeted at the iPhone. “A lot of people interpreted it as a shot at another product,” said Keith Nowak, a Nokia spokesman. “It wasn't its intention, to be honest.”

Indeed, nearly all mobile phone service providers discount the price of a handset, including Nokia's, in exchange for a fixed contract, usually two years. Even some phones sold at full price without contracts remain locked.

Many consumers want to unlock their phones, either with the permission of the carrier or, as is more commonly the case, without it. There are even better reasons to want to unlock a phone using the GSM standard, the most common in most of the world, rather than the CDMA standard, most prevalent in South Korea and Japan.

GSM phones use a small microchip that stores vital information like the customer's phone number and contact lists, as well as billing information. This SIM card is designed to be easily replaced, so someone who wants to switch carriers can simply get a new SIM card from that carrier and put it into his unlocked phone. Another advantage to an unlocked GSM phone is that it can be used in other countries by buying a local prepaid SIM card, thus avoiding huge international roaming fees.

More carriers are coming around to offering to remove the lock after a period of time, such as after 90 days or when the contract is fulfilled. Unlocking a phone without permission may void its warranty, so the consumer must pay to repair or replace it if something goes awry.

Apple said that nearly one out of every six iPhones sold in the United States was bought with the intention of unlocking it. Apple has tried to thwart the practice by updating the iPhone's software, turning any unlocked phone into a brick. But outside programmers say they have already broken the software lock again.

Apple locks its iPhones in the United States to AT&T, and it has announced similar exclusive deals with O2 in Britain, T-Mobile in Germany and Orange in France. Some European lawyers have noted that certain countries require mobile phones to be sold unlocked, and that would eliminate the exclusivity.

Antoine Gendreau, a French intellectual property lawyer, said in an e-mail that a seller must offer the product without a service plan even if he can sell it with a service plan at a lower price.

“A purchase with a service plan that is locked on an operator's network must be able to be unlocked so it can be used on any network at the request of the buyer, and must be done at no charge after six months,” Gendreau said. Orange has said it will sell an unlocked iPhone.

Some cellphone shops advertise that they will unlock certain phones for a small fee, usually $50 or less. And plenty of independent sellers offer unlocked cellphones; prices advertised online for an unlocked iPhone range from $400 to $700, depending on the storage capacity.

Guides have been published on the Web for unlocking popular handset brands like BlackBerry or Treo. Unlocking an iPhone is something that anyone can do in under an hour, said Kyle Matthews, the 25-year-old co-founder of ModMyiPhone.com, who said he had unlocked approximately 50.

“People that have no technological experience would be a little put off by it, because it's a little bit of work,” he said, “but anyone who has basic computer knowledge - it's really easy to do.”

The publicity around unlocking the iPhone has brought the issue to the attention of many Americans for the first time, said Susan Crawford, a visiting professor at the University of Michigan Law School who has studied the legal issues around unlocking.

“This may lead consumers in the U.S. to rise up and demand that their phones be unlocked,” she said.