The term "burner phone" is a slang that refers to cheap, disposable pre-paid cellular phones. Burner phones are often associated with illegal activities; however, people can use burner phones for legitimate temporary phone numbers. Smartphone apps like "Burner" make it possible for someone to use the burner phone concept without having to replace the hardware; the app temporarily forwards calls from the burner phone number.
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Burner Phones in Action
In some situations, you might not want to expose a personal phone number to the public; for example, you could be having an estate sale or selling a lot of property before a move. A burner phone allows you to list a contact number in classified ads, on fliers or on services like Craigslist without worrying about the wrong people calling you on a personal line.
Harder to Track
Burner phone service providers don't track personal data during use or at sale, making the line owner more difficult to track down. However, anonymity is not guaranteed. Burner phones can still be located over the cellular network by phone number; whenever you place a call, the other line's service provider may track the call record. Additionally, the NSA may be able to track burner phones based on new line activation following old line termination.