Reddit mentions of Your Cable Store DB25 25 Pin Serial Port Female/Female Adapter Gender Changer RS232

Went back to my OP and provided the links to Amazon below for each:

Raspberry Pi (pick a flavor but it needs to have an available USB port. Go for a B+ or Pi 3)
Install tcpser - read the docs and install his update, which has some improvements over the APT version.
sudo apt-get install screen (so you can run tcpser in the background)

Now, the hardware:

This “6 Ft Apple/Mac to HAYES Modem Cable” (Mini-DIN 8M to DB25M) link

A 6-ft DB25M to USB RS232 cable (for the RasPi/tcpser option) link

A DB25F/DB25F gender changer link

Lastly, my command line (insert in /etc/rc.local once you test):

su - pi -c "screen -dm -S tcpser /home/pi/tcpser/tcpser -s 19200 -l 7 -d /dev/ttyUSB0 -tsS -i\&k0"

Adjusting for the actual path to tcpser and desired baud rate. The Plus and SE (therefore, everything else newer than these) can handle 19.2k with no problem.)

Also important: plug in the USB cable and "cat /dev/ttyUSB*" to see which USB interface enumerates - should be 0 - but check and adjust if needed.

What you're doing here is opening a screen session, setting to 19,200 bps, pointing it to listen on the virtual USB serial port, debug-level logging and disable flow control (via AT command). If you want to test without screen (and should), just delete everything prior to /path/to/tcpser.

THEN, if this is all working correctly, you need to launch ZTerm/MacTerminal/etc, set the baud rate to 19,200, ANSI if possible and save. You should be able to type 'AT' and get back a familiar 'OK.' Then it's just a matter of going all 'ATDTbbs.fozztexx.com' to get your feet wet.

I would then recommend looking into installing SLiRP on your Pi, which can provide a PPP stack for your "dial-up" machine (like any old dial-up ISP) but start simple.

Good luck!