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I read this article on hacking phone wiretaps and made an MP3 of the C tone, a 852 Hz/1633 Hz hidden DTMF. Play this tone into the receiver and disable a wiretap on your phone line.
Matt Blaze's original article on the C tone is a good place to start. His more detailed paper about the C tone is another good resource.
My phone isn't tapped and if it is, whoever's doing it is wasting their time. Therefore I have no reason to use the C tone myself. It's here just like the infamous mosquito ringtone and the Cap'n Crunch whistle's 2600 Hz tone. I like making sounds.
Is your cell phone being used as a bug without you knowing it?
Let's not forget raw DTMF MP3 files in MP3 format. File this all under phone phreaking.