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?
I have found this article 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.
BTW: this is not telemarketing, it is a notification system. Not that it matters... ;)
EDIT: I learned this system is known by keyword "IVR" (added for future searchers).
Answer
A modem is generally used for data transmission but it sounds like you are actually transmitting voice, albeit prerecorded voice, and tones.
Although you can achieve this with the extra voice capability of a voice modem as you have identified, it might actually be easier to use a GSM to VoIP gateway, unless you actually need the modems data capability for some reason.
You could connect the VoIP to GSM gateway to a low cost or free open source PABX (such as Asterisk) and use this to build your particular application.
Some gateways even support multiple SIMS so you can target the SIM which gives the lowest call rate for the number you are calling - for example if you know that the number you want to call is on a particular operator, then that operator may offer free 'on network' calls between it's SIMs. This might be an advantage if you are expecting to make a large number of calls.
If you do a web search for 'VoIP GSM gateways' you will find some examples.
The following link provides an overview also: http://www.voip-info.org/wiki/view/VOIP+GSM+Gateways