title: Voice call through GSM modem
body: <p>I would like to use a GSM modem to make a voice call to a phone number, play a recorded message, wait for a digit to be pressed and then disconnect the call. The system needs to know if the line was busy, if the user answered and which digit was pressed by user (if any). It should drop the line if nothing is pressed in 30 seconds. If user presses the digit before the question is completed then the voice should stop (user doesn't have to wait). Also, it would be nice if system could handle invalid digits (play "invalid digit" message, play original message and wait for input). Multiple calls in parallel would be nice, but I guess this is not possible?</p>
  
  <p>I have found <a href="http://en.wikipedia.org/wiki/Voice_modem_command_set" rel="noreferrer">this article</a> which explains low-level interface quite nicely. However, this question is more pragmatic - which libraries and which GSM modem would you recommend? OS is Linux, modem can be either RS232 or USB. I would prefer Python, but C(++) is ok too.</p>
  
  <p>BTW: this is not telemarketing, it is a notification system. Not that it matters... ;)</p>
  
  <p><strong>EDIT</strong>: I learned this system is known by keyword "IVR" (added for future searchers).</p>