So to add my two stories:

I provided the Idea and a whole bunch of time/labor/etc to start a dialup ISP in our hometown back in 1994.   I remember having a big debate on whether to bring in a single 56K leased line or 128K fractional T1.  We went with the Fractional T1 just because it could be easily expanded over time.   (That T1 is now multiple 10GB circuits - yes the ISP is still running and I still am involved).   So a single 128K fractional T1, a cisco 2501 (with external DSU, those internal cards didn't exist yet), and 8 14.4 modems attached to a single Sun Unix box.  Note that this was pre-web, and back in the days where you pretty much knew at least generally everything which was on the internet.

Things grew quickly, don't remember how many lines.   At some point we moved to having 56K modems on our end, which required a digital carrier to the central office.   T1's were very expensive, so we did a bit of tariff arbitrage.   One could obtain a 'metered' ISDN BRI line for like next to nothing - the metering had to do with the fact they were going to charge you by the minute for any calls, but here's the catch:  for outgoing calls only, incoming calls were free which worked great for a dialup ISP.    The problem was that 56K dialin concentrators all wanted T1 lines.    What we discovered is that Adtran made a box which would take a whole bunch of ISDN BRI (each with 2 channels), and combine them into a single T1.   And due to the retail pricing difference for T1 vs BRI, we could pay for the box in a few months.    So we took a whole truckload of ISDN BRI lines and combined them into a few channelized T1's and ended up paying a lot less to the phone company.

Of course, things have grown past that (we have an extensive WISP network and have an ever-growing amount of fiber in the ground).  But it's fun to think about where we started.