As mentioned, the ISDN is/was a guaranteed bandwidth service (really a partial T1 line)
No, ISDN is actually a dialup service, though digital from end to end and not analog. 64 kbps per channel, two channels available per twisted pair. I used dual-channel ISDN to provision my first hosting company. From a single twisted-pair connection. I ran it from an apartment with two twisted pairs available, and used one for a fax line (and occasionally a modem for testing). The other twisted pair was for a dual-channel ISDN line, which switched automatically from 128 kbps down to 64 kbps whenever a phone call came in.
My upstream provider also worked from home; he had a partial T-1 (he never told me how big a part) and I paid for the ISDN at my apartment and at his home.
This worked because in Florida (where I lived at the time) the local tariffs were for flat rate for residential ISDN. If we worked from offices we couldn't have afforded the per-minute rate. The relatively slow speed and capacity worked in those days (circa 1995-1996 iirc) because the local university only had 56kbps for the entire campus.
Years later running an ISP I had a 25-pair cable, which we used for twenty five incoming phone lines to 25 separate 48-baud modems, and offered dialup service. We didn't pay incoming at all, so the charge was under $500/month for incoming data. Outgoing to the 'net was 384kbps sDSL at a few hundred dollars per month (one twisted pair).
That model stopped working when 56K modems came out; they would only work at 56K downstream, not upstream, so they wouldn't help us provide 56K service. So we dropped our physical plant and began reselling a wholesale ISP who'd invested in the newer equipment.
That turned out to be a good plan; a few years later when everyone was switching to DSL (before the days when you could get Internet from your cable provider) we sold off our clients at a profit, didn't have a great loss from equipment fast becoming obsolete, and moved on to a hosting-only model. Our one competition in town, the local daily newspaper, lost a fortune; they had to write off a much larger infrastructure.
Those were the days.