I needed a way to extract audio from a phone line for my DTMF decoder. I found a site full of examples on how to do it, and I built an interface that suited my needs: RCA output, with a signal strong enough to be fed directly to an audio amplifier or line-in. The interface can stay connected on the line at anytime (no problems with the ring signal), and does not take of the hook. The interface must provide isolation from the line. I discovered that it can also be used to inject audio in the line.

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