I dunno about your modem if it is voice capable, but if it is and your soundcard has either an aux or modem audio input jack (internal) and the modem has an audio out jack (internal), then connect these jacks with the right type of cable and enable the modem or aux audio input on the soundcard.
If the modem is voice enabled and doesn't have an internal connector, it may have an external audio in/out mini jack (3.5 mm). If it does, connect the audio out jack to the sound card line in jack and enable the line in audio on either your mixer or the soundcard itself.
Lastly, if the modem is voice enabled, but doesn't have either external or internal connectors for audio in/out, then it must do voice passthrough in hardware/software. You would need to find out if the software to pass the voice audio through to the soundcard is 2000/XP compatible.