I think we used RocketPorts for a while until switching to Livingston Portmaster 3s, which you plugged a T-1 into.

Portmasters were very popular. Later on the ISP I worked with moved on to Ascend boxes which had digital modems (T1 / PRI lines.)

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.