ISDN did have multiple channels, in Germany you would get two data channels (B1 and B2-channel, 64 kB/s), and the line has a D-channel (something like 4kB/s) for network purposes. First you could bundle the B-channels to get twice the speed, at twice the price and the land line was busy if one did that. That feature got me into trouble with my parents quite regularly.
Second someone figured out that the D-channel can also be used to send data to arbitrary endpoints in the phone networks and dropped a messenger app on a Friday afternoon in '98 or thereabout. That was a pretty fun weekend, unfortunately the hack had stopped working when I came back from school on Monday.
Oh yes, the D channel, holy grail of always-on dreams. Young me spent countless hours staring at AVM header files dreaming of finding an angle that would enable me to sneak free bytes to a friend who also had ISDN.
I did my training at Deutsche Telekom, and our instructors told us a story of an ISP that tried to sell a flatrate working over the D-channel; apparently they had some reliability issues and did not last long. That must have been in 2003 or early 2004, by then of course DSL had arrived and rendered such experiments (and ISDN in general) more or less pointless.
Same here in the Netherlands, somewhere between 1998-1999 using KPN Telecom. I was in high school and convinced my parents to upgrade from single-line PSTN to dual-line ISDN, so we could call and surf the internet at the same time. At the start of 1999, because of the rapid increase in phone and internet traffic, the national call tariffs had been reduced a lot[1]. Tariffs were even cheaper in the afternoon and during the weekend.
For normal internet browsing the single ISDN connection (64 kbps) was enough. Then later in 1999, Napster[2] launched and I was hooked! This made me use bundling ISDN channels quite a lot. Because of the cheaper tariffs, weekends where great because I enabled the bundling around midnight until early in the morning, so my parents wouldn't notice. :-)
I was on ISDN until 2002, when ADSL was available here and we could be online 24/7 for a fixed price. I do remember my ISP (XS4ALL) still had a fair-use-policy for downloading, which they removed a few years later.
When we first got Internet at home (also Germany), it was ISDN, but only single channel. So being connected meant not being reachable by phone. That meant my mom strictly limited our Internet access to 1-2 hours a day. I was so glad when we finally got that second channel :D
Second someone figured out that the D-channel can also be used to send data to arbitrary endpoints in the phone networks and dropped a messenger app on a Friday afternoon in '98 or thereabout. That was a pretty fun weekend, unfortunately the hack had stopped working when I came back from school on Monday.