A determined group of hobbyists has documented breaking the iPhone's ties to AT&T through a mixture of hardware and software, proving that the Apple handset can be hacked to permanently function with other cellular carriers.
Calling their project Finding JTAG after the Joint Test Action Group standard used to test access ports on circuit boards, the hobbyists claim to have refined a surefire but dangerous ten-step process that allows the iPhone to use an unmodified SIM card from T-Mobile or other GSM cellular networks.
The technique requires an iPhone that has already been "jailbroken," or derestricted to allow third-party programs, as well as soldering tools and wiring. Similar to the process for unlocking a Siemens phone from Europe, the process involves forcing the read-only boot memory on the iPhone to accept unsigned code on the phone's built-in NOR flash storage that controls some of the most essential functions. This permits the code to change the iPhone's default behavior, which normally bars all but specially approved SIM cards from placing and receiving calls.