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. Although AT&T, Sprint, T-Mobile and Verizon are the only authorized iPhone carriers in the United States, unlocked iPhones can be used with other carriers. 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.). 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, a lucrative market before the iPhone 3G's worldwide release.