The technical knowledge needed to unlock a modern mobile phone is daunting for some. As a result, one company is actually offering to unlock a competitor’s phone for customers, according to BusinessWeek on Tuesday.

Unlocking a mobile phone from the original carrier is something that only about 5 percent of customers do. However, for those who may want to do so, especially for a used phone, market trends seem to be loosening the grip carriers once had.

MetroPCS operates in Los Angeles, Philadelphia and Detroit. In June it began to publicly offer to unlock phones sold by a competitor, according to Olga Kharif. That’s going to upset the competition, and historically, carriers have locked their customers in with hefty early termination fees. That hasn’t been well received by consumer groups, and the FCC is pondering required changes when the phone isn’t subsidized.

Some carriers have tried to force their customers to remain with them under the contract terms of service and keep them from selling their phone, but that may not be enforceable, according to Robert Bramson, a partner at Bramson, Plutzik, Mahler & Birkhaeuser in California. Moreover, when the contract period is finally over, there’s no legal reason why a customer shouldn’t be able to put that used phone on a new network, Mr. Bramson believes. In an era of valuable smartphones, with a lot of data accumulated, the desire to unlock that old phone is likely to grow.

The situation has implications for iPhone users upgrading to a new iPhone 3G. Apple and AT&T have said nothing about what the customer must do with that old phone, no longer under contract. Of course, they’d be all to happy to add it to an AT&T family plan, but ultimately, it’s up to the customer what they want to do with it.