My Mega ST 4 "Low" uses a ready-built Wi-Fi modem based on dhansel's WifiModem firmware, an ESP8266 module that connects a serial port to Wi-Fi. With it, the machine can dial into telnet-based bulletin board systems using its own terminal software, recreating the dial-up BBS experience without a real phone line.
dhansel's WifiModem is firmware for the ESP8266 that makes a serial port talk to Wi-Fi. It does this in two ways, and they're mutually exclusive:
For dialing out to BBSs, the modem emulation mode is the relevant one. The module understands Hayes-style commands like ATDT followed by a hostname or IP address, and can even limit the line speed via the S37 register for an authentic slow-modem feel. Since the ESP8266 only has RX/TX pins, there are no hardware control lines, but the module handles the "+++" escape sequence to return to command mode during a call.
To put it to the test, I dialed into the DarkForce! BBS, an Atari-centric system that has been running since the early 90s on a Mega ST4 with BBS Express! ST software. It's a fitting target for an Atari ST, since it's a fellow Atari machine at heart. With the terminal software on the Mega ST sending ATDT to the Wi-Fi modem, the connection to darkforce-bbs.dyndns.org over Telnet worked, and the ANSI login screen, message boards and file sections of a real BBS came up on the original hardware, no phone line involved.
The obvious question is "why bother?", the Atari ST was never designed for networking, and a modern device does all this far more easily. But that's not the point. There's real satisfaction in making old hardware do things it was never meant to do, and in experiencing the dial-up era's BBS culture on original hardware, over Wi-Fi instead of a phone line.
If you're chasing BBS nostalgia on real hardware: budget far more patience for the networking layer than for the actual computer. The 1980s hardware is the easy part; convincing 2020s Wi-Fi and modern BBS ANSI dialects to cooperate is the real challenge.
Jungsi tested a WiModem232 OLED on his Atari ST and wrote about it on his blog. He used a different modem hardware than I did, but the idea is the same. He used "Connect" as his terminal software on the ST, but I used a different terminal program that could render ANSI graphics in ST Medium resolution, though I'd have to look up which one it was exactly.