In the US most ISDN service was terminated to an PRI T1. That hooked into a modem bank that would answer both ISDN and 56K calls. By that point the ISDN modems where smart enough to be able to use both 64K channels as a pair bond. It would utilize two modems at the NOC.
It was also pretty common for us to order channelized T1s for 56K applications. You could get one extra modem per T1, but we would lose the ability to get real time ANI data and ISDN.
Eventually we started having the NOC/POPs configured with a T3 and them demux it into a variety of T1s and fractional T3. For instance for a mid size POP we could have a single T3 handle both the incoming phones and the upstream data. I.e. We wanted to drop a crap ton of modems into the back room of a rural shopping mall because that was the most cost effective location based on LATA lines and what was considered the "local calling area".
A lot of the times people would claim MSN or AOL or whatever was the best service. In reality in most parts of the US all the main national services would terminate to the same place and run on the same equipment. We'd just route it differently based on the number you called.