There are some similar scripts out there for "receiving" like Tk-Voice, an answering machine. Just google for them and you ought to be able to gleam clues from them.

Like the other posts say, it is really dependent on your hardware. I would think along these lines.

Connect the line-out from your sound card to the audio-in on the modem. Use Device-Serial Port to gain control of the modem, use Audio's "dial" script (or something similar) to send the numbers tones. The hard part is detecting an answer :-)

Once you determine you have an answer, use audio to play your wav file.

Device-SerialPort has quite of few examples to get you started.