Simple minimodem BBS with a 3d printed acoustic coupler case. The acoustic coupler was designed to hold a USB speaker and microphone, which you can buy from Adafruit.

Requires Linux. PJSUA and minimodem must be installed. The BBS uses shell scripts and javascript to relay messages via minimodem over VoIP. On the server side, start PJSUA then start the phoneMonitor.sh script. Minimodem is set to run at 100bps. Edit the shell scripts if you want to change the baud. You will need to edit the clientSide.sh script, minimodem -A alsa option should match your USB microphone and speaker. Use arecord -l and aplay -l linux commands to find their card numbers.