ISDN was the standard for "next generation" phone lines. It was never really meant for data. The design was to handle converted analog signals their new digital network. The demand for true broadband upended that plan, tough, and not it's just voice over much faster IP technologies.

1200 bps modems were available for the IBM PC much earlier than that. I think by 1982 your could get a 1200 or even 2400 bps modem for the PC. The AT models in 1984 standardized the faster speeds.

300 baud modems were still popular in the 1980s, because most people still used home microcomputers, and that's what was available for those low end machines. Atari even sold an acoustic coupler modem for their 8-bit computers that was very popular as it was quite inexpensive.

I worked at Radio Shack in the early 1990s. I was shocked to see that Radio Shack was still selling the Model 100s (acutually, model 102 by that time) as it was at least a 10 year old design by then. The manager told me that journalists still liked to buy them, because it was rugged, super portable, ran on batteries, had a great keyboard, and they could upload their text over a phone line. All I can say is their articles must have been very short as the memory limitations of the device were severe.

I might not remember this right, but I don't think that the memory limitation was a factor, as they were often used with rudimentary dialup terminal software as a dumb serial terminal dialing in to a newspaper run system. The story would sometimes be typed live from the reporter's notes directly into the shell session of the remote (minicomputer, etc) system on the far end. This mean that even if there was 5-8KB of text it wouldn't be a problem.

Then any formatting/line wrapping/typo and other mistakes from the raw submitted story would be fixed by copy editors from their local workstations.