You can telnet to many BBSs. If you want something a bit similar to the original modem experience, you can buy device that emulates modem but actually uses WiFi and connects to telnet instead of a phone number.
I just built one of those today, the Wirsa v2 (its on github) and it's great for getting older devices "online". You can telnet, or playback text files, or download files from sd card all via a DB9 serial - you can use a DB9-DB25 converter if you want. I got it so I can get my TRS-80 Model 100 online, and maybe even an Epson HX-20, but I still need to build a cable for it.