Manual for the modem in question >>> http://support.dell.com/support/edocs/comm/p48210/en/index.htm << not a regular PCI card modem, more of an expansion daughter board for the mainboard....
My friends modem has a green connector on the side of his modem that he can plug into the side of a sound card but mine does not.On the modem card itself, there should be a connector similar to that for the audio out of the CD player. This can be connected with a jumper to the line in on the sound portion of motherboard, or sound card if system is pretty old.