Every cell phone comes with a unique International Mobile Equipment Identity number, or IMEI. If a phone is stolen, the service provider will use the IMEI to prevent the stolen cell phone from making calls, known as blacklisting. The service provider also will blacklist this number if individuals have failed to pay their bills. If you buy a used cell phone, and you don't check to see if it's blacklisted, you may have to go through the process of unlocking your cell phone.
Call customer service for your blocked cell phone. Tell them that you've purchased the locked cell phone and would like to have it unlocked.
Prove your identity by using a Social Security number and mailing address. If you are unable to do this over the phone, ask for any locations near you that can help verify your identity.
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Go to your service provider's store and explain to the representative that you've purchased a locked cell phone. Give a representative your information. The rep will change the SIM card on your cell phone, allowing you to access it. The exception to this is if the phone was reported as stolen or lost, in which case you will have to surrender the phone.
Get your money back from whoever sold you the phone if you were unable to unlock it.