How could you transfer data over an audio line?
Here is an example project using FSK: http://sree.cc/electronics/arduino-as-an-fsk-modem


  An FSK modem basically convert a digital signal into analog signal or
  sound signal by frequency shift keying technique. Here we can make our
  Arduino as an FSK modem by uploading a simple code. This project
  produce and play a sound corresponding to a digital signal and
  demodulate a sound signal into the corresponding digital signal.