If you’re connecting from a modern computer, you just get a telnet client that does the appropriate code pages/ANSI/zmodem/etc. If you’re connecting from a real vintage computer, you get a little dongle that pretends to be a modem (and often accepts AT commands, including fake phone numbers), but secretly connects to WiFi and relays through a telnet connection.
Some BBSes do still have landlines, and there’s the occasional ham radio BBS, but 99.999% of it is through IP-based telnet or ssh these days.