I don't recommend plugging residential twisted pair phone line directly into a mixing board or sound card. A ring (ya know, someone calling you) can cause 40 to 60 volts to be generated. Even when you are not using the phone there is a nominal voltage of 4V or so. Oh, sure, the current is VERY low, but the voltage is more than enough to fry the typical sound card. Anyone interested in using this method should get an isolation device from Radio Shack, Jameco, Mouser, or your favorite electronics supply house. These devices will not only filter out the voltage jump on ring and nominal on-hook voltage, but will also provide correct impedance matching between the sound card and phone system.
Just here to help!