Cell phones are blacklisted if they have been reported as lost or stolen to the local phone provider and the phone's International Mobile Equipment Identity (IMEI) is entered into the Central Equipment Identity Register. Older phones used to only have the main cell phone usage disabled, but handset use is now blocked as well. For the past few years, all cell phones allow blocking the handset to prevent cell phone thieves from being able to use them. The only way legal way to unlock a phone is to contact your cell phone service provider.
Call the customer service number for the blocked cell phone provider and tell them that your cell phone is not stolen. If you recently bought the phone you are using, tell them.
Confirm your identity over the phone with the provider with an account number, cell phone number, Social Security number and mailing address.
Ask which of the provider's stores near you can offer technical support for your blacklisted cell phone if you are unable to get your phone unlocked over the phone.
Bring your cellphone and account information to one of the provider's stores that offers technical support if you were unable to unlock the phone over the phone with customer service. They are likely to change the SIM card on your cell phone. If the phone was reported lost or stolen by its original owner, then it must be surrendered to the provider or cell phone's original provider.
Report the business you bought the phone from if it was reported lost or stolen to the police. If you bought it from an online store such as eBay or Amazon, report the problem to them as well to get a refund.
If you did not buy the blacklisted cell phone at an authorised store, it is likely that you may have purchased a stolen phone.