I'm not sure if this is even possible - I'm no telephony buff - but is it possible to record telephone conversations using a DSL/Ethernet connection? My line comes from my wall, then splits to my DSL modem (modem > router > computer) and a telephone. I'm guessing that the voice signal is there, but it's just disregarded because the frequency is too low to mean anything to the hardware. Is this right? and if so, can I capture the voice?

I know that such a thing could be used for illegal purposes - I plan on using this to record conversations w/tech support people etc. so that I can have conversations for future reference.

There are several howtos on setting up an answering machine but you will need a dialup voice MODEM. Search for mgetty and vgetty. Most dialup modems these days are winmodems which depending on the chipset will determine if it works with linux. Your correct the DSL MODEM ignores voice frequencies.

http://linmodems.org/