The term "blacklisting," as it relates to cellphones, has several different applications, but is typically used to refer to a cellphone that has been stolen and reported as stolen. If a phone is blacklisted for any number of reasons, it will be impossible to use. You can also create your own "blacklist" of phone numbers that belong to telemarketers.
If your cellphone is stolen, it's important to contact your service provider as quickly as possible to report the crime. Your provider will be able to blacklist your phone and number, meaning the thief will no longer be able to use it to make or receive calls. Additionally, the service provider will often give a report of blacklisted phones to the police so that when phones are recovered, they can be matched up with their proper owners.
If you've lost your cellphone, and you don't want someone to find it and be able to use it, you can call your provider and have it blacklisted. If you're able to find the cellphone, you can then call your provider back and get your phone removed from the blacklist. While it's on the list, it won't work, but as soon as you provide identification and request that it be removed from the blacklist, you'll be able to use it again.
If you buy a used cellphone, it's important to try it first to ensure it isn't blacklisted. A thief may try to sell a stolen cellphone to an unsuspecting buyer. To test the phone, try to make a call with it. If this doesn't work, use another phone to call it, then monitor how the phone in question works. If the phone is blacklisted, it may ring once, then display a "Missed Call" message.
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If you get regular calls on your cellphone from telemarketers, surveys and other unsolicited numbers, you can block them to create your own "blacklist." Though the process of blocking an incoming number varies depending on your specific cellphone, most phones give you the option of blocking a number. Over time, you may add several numbers to this list, which could be called a "blacklist."