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.
I found many examples on this site Telephone line audio interface circuits. The schematic of the phone audio interface I built:
I have added a switch to choose between a 620 ohms and 33k resistor. This attenuates the signal coming out from the interface, provided there is at least a small load(10k works fine) between audio out and audio gnd.
Important: This circuit works in Canda and probably in the United States. It has not been tested in europe, but it may work if the phone system is similar enough.