Tapping modem transmissions

It's not too technically difficult to tap modem connections at low speed, I played around with it a few years back, and I was able to add a third modem onto a 300 or 1200 baud connection and view what was being transmitted. Using the modem as a dumb interpreter of the signal works well to analyze the raw data being transmitted at low speeds, but when you use v.42 and LAPM, things get a bit more complicated. You need special hardware/software to interpret and decode the compressed & checksummed data packets.