I don't think it's practical. The lossy compression that GSM uses is incompatible with the protocol used with modems, which assume the analog characteristics of real phone lines.
If you want to reach a BBS on your phone, you could use a telnet app like Termius or Termux over 4G.
There's certainly practical issues, but not every call is stuck with the GSM codec anymore. G.722.2 (aka HD Voice) may be better than GSM at carrying modem noises (or it may not be).
Of course, way back when, when mobile carriers had modem banks for outgoing calls, that worked a whole lot better.
Take a look at the Quiet Modem [1] project, which has bindings for Android and iOS if you're looking for a softmodem. I'll bet there's telnet apps on iOS though if you just want to do some IP based BBSing.
I remember someone trying to port minimodem to run under termux. Nevertheless, it still not a HAYES modem, but I don't think it is difficult to build one using minimodem.
Readit News
'When we went for a new wife
acquaintance, on the way I asked "What is she doing?" .. He replied,
slightly embarrassed, "Yes, nothing at all."'
http://www.bbsdocumentary.com/
https://www.telnetbbsguide.com/bbslist/ibbs1121.zip
Anything like that for Android?
If you want to reach a BBS on your phone, you could use a telnet app like Termius or Termux over 4G.
Of course, way back when, when mobile carriers had modem banks for outgoing calls, that worked a whole lot better.
People say this all the time on the internet, but in the 90's, I had a Nokia phone that could send faxes over GSM.
[1]: https://github.com/quiet
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And Nethack/Slashem/Crawl over Telnet/SSH, OFC. A must have.