Just want to let you know that if you direct-connect a speaker or headphones to the phone line, there will be an audible 'click' on the line, just as if someone picked up another phone on the same line. Also, it will open the line if there is no call.
If I remember correctly, it's just the green and red wires.
To tap into the line without the 'click', you need an induction tap.
Without that... you would need something (an alert homo sapiens often does the trick) to connect the device when someone else picked up the phone. I like the HS method because it's cheaper, but it only works if you're in the same residence and can tell when someone picks up the line.
Back off, man! I'm a scientist... - Peter VenkMann