I'd just copy everything to tape as I'd been doing for years. You don't need anything new to tap the phone line to pick up the data. After that, you just need to figure out the baud rate, parity, and data compression (which could be done by trial and error pretty quickly) and then just play the tape into a modem that was set up correctly.
Unless I'm missing something, it seems pretty simple.
I have heard that tapping high speed modems can be a little tricky because of the adaptive filtering done. The modems both have adaptive filters that adjust to the line as they see it. If you are tapping at some other point the filtering will not be optimimal for your case. I believe these filters take care of smearing and echos and things of that nature. Maybe someone in the know can elaborate and clear up all the bad data I probably just spewed :) Phil?
-David