I have an idea for a new shield for the enterprising bunch out there.
A telephone interconnect shield.
It would have the required circuitry to detect ringing, some method to allow the arduino to pickup the line as well as DTMF encoder/decoder.
I have many remote monitoring/control ideas that need telephone inter-connect. It would be nice if I did not have to hack an old modem or try to do the opto stuff myself. It would be nice if the shield had an easy to find audio insertion point in case we want to feed an audio source onto the phone line under micro control.
I am not sure how to generate voice without the expensive SpeakJET chip (expensive if included in the basic telephone shield).