I don't think this is the right forum.
The difficulty is getting software to act as modems. Nowadays, there is no mainstream market for that.
I would point out that internet connectivity doesn't require broadband, and if you are in a country which has areas to which broadband doesn't reach, your ISPs really ought to still support analogue modems. I'd therefore suggest that the easiest option is to have one line bypass Asterisk and go direct to a real hardware analogue modem. My feeling is that anything else, although the software may exist, will require you to hire someone with quite deep technical skills, as they will be dealing with technology that is no longer fashionable, at a quite deep level.
Also, if you are somewhere with no broadband, it is quite possible that the analogue telephone network will only support quite slow modems.
You will not be able to attain more than 33.6kbps point to point, for pre-compressed data, even if the network is good. You might be able to achieve 56kbps downlink from an ISP, but you would have to use a hardware modem directly on the incoming line, or implement the modem entirely in software, if going through Asterisk (or any digital PABX). This would also depend on your local PSTN exchange being digital and being connected to the rest of the country, digitally.