I used to know a couple of people who used dumb terminals and couplers to connect to CompuServe. One guy used one of those little Radio Shack portables. I even had an account there myself. You could do almost anything once you connected - talk on forums, play multiuser onine games, send messages to other CIS users and some other systems like CIS, store files in your own secure area, share files with others, and download from public areas. You could search the US Copyright Office and some other public databases, or follow the gateway to Lexis/Nexis, or you could even send FAXes, or use the online word processor and have your letter printed out on a letter-quality daisywheel printer, folded, put into an envelope, stamped, and mailed to someone offline. Except for not having inline graphics, you could do most of the stuff people do with the internet now.
The whole thing ran of H&R Block's mainframe farm, and they charged a per-hour connect fee, plus additional fees for other services. All you needed was a dumb terminal and an acoustic coupler, though you could use one of those newfangled "Personal Computers" and fancier software if you wanted.
Compuserve's competition was BIX, GEnie, and Prodigy, though they were all eclipsed in the end by America Online.
All AT&T or Bell would have had to do was set up something similar, and let you add your subscription fee to your phone bill. But they were in the "provide a dial tone" business, not the "do something with the dial tone" business...