A locked phone is a phone that's restricted to working with one service provider only. That's why a T-Mobile customer, for example, can't just stroll into an AT&T store and walk out with a brand-new phone without also switching carriers.
To many customers, this predicament isn't a big deal, as all they want from a phone is reliable service and a few synchronized functions. But others aren't quite so optimistic and believe that once they own a phone they should have the freedom to decide how to use it. If you're in the latter group, consider buying an unlocked phone as an alternative.
At its core, an unlocked GSM phone is simply that: a phone that's not locked into a single carrier's network. In other words, you can choose your carrier and phone independently, albeit with a crucial caveat--unlocked phones only work on GSM-based networks, which effectively eliminates a couple of major carriers from consideration (Verizon and Sprint).
The technology is simple from the consumer's perspective: GSM phones contain a small, removable card, known as a SIM (Subscriber Identity Module) card, that's programmed with your subscription information and phonebook. If your phone is unlocked, you're free to remove your SIM card and insert it into another unlocked phone without losing your contacts or carrier data.
Unlocked phones aren't a new phenomenon, but until recently, consumers could buy them only in "gray" markets (i.e., markets where phones from other countries were sold--mostly phones from the European and Asian markets). That's changed as of late, and now manufacturers are selling unlocked phones in the United States in far greater numbers.
That's an important detail to understand--you don't always need to buy a new unlocked phone if you're happy with your existing GSM phone; instead, you can have it unlocked. Although it's sometimes possible to do this yourself if you know the unlocking code--a number that corresponds to each cell phone--many people opt for a postal unlocking service.
In the latter case, the user sends the phone to a specialist, who makes the modifications and sends it back with a damage-free guarantee. This is typically the best option for non-expert consumers, as some brands require extensive hardware modifications that can easily harm the phone if performed improperly.
