In the telecom world, the telecom was the network. If you wanted to exchange data with someone, you placed a call to someone directly. Everything was envisioned as some form of point to point session between subscribers. Whatever it was, it was one subscriber contacting another. The telecoms didn’t really conceive of what would become the Internet as we think of it now. As far as they were concerned, the Internet didn’t provide anything the telecoms couldn’t offer—and bill for.
They were caught flat footed when customers started dialing up ISPs and cutting the telecoms out of the loop. If you wanted Internet, you got ISDN, but you used it to call an ISP that was not the phone company.
Not that the telecoms weren’t visionary — they largely foresaw everything we do today. They just assumed they would continue to be the sole provider of those services. They built a network with that in mind.
And then we all went and built our own network on top of that.
They scrambled to try to charge us for using our voice lines for data, and then they managed to slide DSL onto the rate card, and ultimately became actual ISPs.
But that wasn’t the future ISDN was built for. It wasn’t built for a world where an ISP was even a thing. It was built for a world where everything we now do on The Internet would show up as a line item on a bill from AT&T.