My ISDN was sold as "ISDL" by an ISP. Still had the performance you're describing, but it was tied to them. There was no dialing on my part: it was just always up. I'd pay for it today if an ISP offered it at a low cost, as a backup.

I missed the IDSL phase completely. I'm not even sure if it was ever available in my neck of the woods.

For me, it the continuum went like this: Dialup > ISDN > dialup > slow DOCSIS > faster VDSL > faster DOCSIS > [this is the part where I write a whole chapter about how there is fast, cheap gigabit fiber available in rural areas directly surrounding my small city, from multiple competing companies, but none within the city limits]

Anyway, IDSL. That technology skipped right by a lot of what was cool about ISDN. For me, real ISDN was always-on unless I disconnected it for some reason. While still "dialup" in the strictest sense, it was not infrequent to have sessions that went for months without any interruption at all. But I could also do anything else I wanted with it.

And backups: Apparently these days, a person can get a slice of Starlink pretty cheap. In this mode ("Standby Mode," IIRC), it provides a slow, always-on connection -- I think it's $5 per month for ~500Kbps.

The RV and snowbird communities hate it because it isn't free (they used to be able to pause service in off-season without monthly cost), but it sounds pretty good as a fixed, domestic backup: 500kbps is a lot more than 0. (And if this backup needs used for a long time or speed is important, then: 500kbps is way more than enough bandwidth to log in and pay for a month of real service.)