Recording land line phone

There is about 40VDC on the phone line when the phone is on-hook and that voltage drops when off-hook so you should be able to detect that voltage change.

You need to protect/isolate your electronics from the 40V and also from the 90VAC ringer voltage. And, be careful when touching the wires because if a call comes-in, the 90V ring voltage can be "unpleasant".

[u]Here[/u] are some devices for tapping-into the audio signals.

You'll probably need a preamp and you might want to try recording some "normal" audio first, such the line-output from a CD/DVD player or the line/headphone output from your computer etc.