Although one offers only prepaid service and the other both prepaid and post-paid contract service, cell phone providers Boost Mobile and T-Mobile both use "GSM" technology -- which involves the use of SIM cards -- in their cell phones. T-Mobile usually take measures to make SIM card switching more difficult -- namely, by selling phones "locked" to its network. If you purchased an unlocked T-Mobile phone, however, it's possible to use it with your activated Boost Mobile SIM card.
Charge your unlocked T-Mobile phone if it doesn't have adequate battery power and power it on.
Remove the phone's back and battery and pull the tray off the SIM card slot, which is under the battery. Remove any existing SIM card if there is one. Insert your Boost Mobile SIM card into the phone face down. Replace the phone's battery and back and power it back on.
Wait at least 30 seconds -- and up to a full minute -- for the phone to recognize the Boost Mobile SIM card. Keep an eye out for "Boost" to appear in the upper-left corner of the cellphone's screen, alongside "bars" that indicate the strength of carrier signal. Begin making phone calls and sending text messages on the Boost Network.
If you haven't yet activated your SIM card with Boost Mobile, visit the Boost Mobile website and click the "Activate" link at the top of the page. Once you have activated your SIM card and set up a Boost Mobile account, it works in your unlocked T-Mobile phone.