If you're tired of your current Sprint cell phone and want to switch to a new phone without signing a new contract, all you need to do is switch the SIM card from your old phone to a new one. This will cause the new phone to register as your old phone and keep you on the Sprint network in your current contract.
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Remove the battery cover from the back of your Sprint phone. Depending on the model, it will either slide off or require you to press a small release button to pop it off.
Remove the battery from the phone to access the SIM card slot below it. Slide the SIM card out of its slot. This may take a few tries, as the SIM card is small and may be a bit slippery. Place your finger on top, push down and slide the card out. If you cannot get the card out, try placing the edge of a credit card at one end of the SIM card and gently pushing to slide it out. Make sure to push in the direction the card needs to slide to release it or else you may jam it further into the phone.
Insert the SIM card you removed into your new phone. Insert a different SIM card into your old Sprint phone if you plan to use it as well. Replace the battery and the battery cover.
Turn your new phone on and wait for it to power up. If you get an error message, the phone you switched your SIM card to may not be unlocked. Call Sprint's customer service and ask for help unlocking the phone. While some phone companies may hesitate to do this, if you are staying with the Sprint network and simply switching your SIM card to a new cell phone, the carrier should have no problem giving you the unlock code and walking you through any extra steps you may need to take. Many new cell phones come unlocked automatically and won't require you to enter any unlock codes.