Boost Mobile, a prepaid-only affiliate of Sprint, allows you to activate used phones to its network if you don't want to purchase one directly from Boost, provided the handsets meet certain requirements. In some instances, it's possible to activate an Apple iPhone to the Boost network, although the carrier doesn't officially support the iPhone.
GSM vs. CDMA Technology
Boost Mobile is atypical for mobile carriers in that it accepts both GSM -- in other words, SIM-bearing -- mobile phones, as well as phones that don't require SIM cards and use CDMA technology. Not all GSM phones work with Boost SIM cards -- they must be "unlocked" to work with any carrier, a procedure that voids the device warranty in the case of the iPhone. For CDMA devices, the process involves additional steps.
Flashing and Activation
While unlocking a GSM iPhone is something you need only do once -- once the device is unlocked, it can be used with any carrier at any time as long as you don't re-lock it -- flashing is specific to each carrier. As a result, before you attempt to activate a CDMA iPhone to Boost, you must "flash" it, a complicated re-programming that usually requires a professional. Like jailbreaking, flashing is unauthorized by Apple.
Unlocked GSM iPhone on Boost
If you already have a jailbroken iPhone -- or have successfully jailbroken the device yourself, in spite of knowing it voids Apple's warranty -- you can use Boost service on the account. If you don't already have an activate Boost SIM card, order a "Boost Starter Kit," which includes a SIM card, from Boost's website.
CDMA iPhones on Boost
If you're able to flash an iPhone to Boost's signal, you can activate the device online via Boost's website without purchasing any sort of kit, as you would need to with a GSM iPhone. Keep in mind, however, that since Boost doesn't officially sell or support the iPhone, you may be left without technical support if the device suddenly fails when you use it with Boost.