MajorBBS could handle multiple lines on its own, but you had to handle ALL of the lines with one box. That meant a serial port interface like DigiBoard which provided some number (8 or 16 or more) of serial ports that you would connect to modems.
PRI was a huge step. The "individual modem" days were a mess. Each modem had a serial cable, phone line, and power brick. I remember doing some maintenance in one of the POPs. There were at least 100 modems, stacked on a cheap plastic shelving unit. The shelving unit was sagging from the weight and heat of all the modems.
This early POP was haphazardly built, so no cable management. I remember a river of phone cables coming out of the wall. The power bricks were also crazy. We had power strips 2 or 3 levels deep, making it a hazard to even get behind the rack without tripping on something.