While the iPhone was initially sold in the U.S. only on the AT&T network with a SIM lock in place, various hackers have found methods to "unlock" the phone from a specific network.[203] Although AT&T, Sprint, T-Mobile and Verizon are the only authorized iPhone carriers in the United States[citation needed], unlocked iPhones can be used with other carriers.[204][failed verification] For example, an unlocked iPhone may be used on the T-Mobile network in the U.S. but, while an unlocked iPhone is compatible with T-Mobile's voice network, it may not be able to make use of 3G or 4G functionality (i.e. no mobile web or e-mail, etc.).[205] [failed verification] More than a quarter of the original first generation iPhones sold in the U.S. were not registered with AT&T. Apple speculates that they were likely shipped overseas and unlocked (also known as box breaking), a lucrative market before the iPhone 3G's worldwide release.[22][206]