I worked at a couple of small ISPs in the mid 90's, and built the dialup infrastructure of one from scratch. I remember using Xylogics Annex terminal servers connected to either 16 or 32 individual USR modems. There were stacks of modems, each with its own power brick, serial, and RJ-11 connection.

This was a few years before everything went digital with PRI lines (T1s that let you do digital modems, basically.)