I was installing Windows 2000 on a PC in the lab (manual disk swapping required) when, hidden behind another rack, several shelves full of physical modems all started calling the box at once, speakers on.
These were great boxes, and the only way you could get 56K was to call into an ISP with one of these or similar on their end -- the trickery that allowed 56K relied on one end being fully digital.
I was working for an ISP around that time and we had a bunch of Portmaster 2s connected via RS-232 cables to piles of modems, some rackmount some just stacks and stacks of US Robotics Sportsters. Sometimes modems would get wedged and we'd have to reboot them or "busy out" the line that they were on. Harder for the modems that were an hour away.
When the transition happened we were able to get rid of all those wires and just plug in one small phone cable for the T1, another for Ethernet, and terminate 23 lines. The Portmaster would treat all the modems as a pool and route calls to whichever was available, and once a call was done would run some testing on the modem before putting it back into the pool. It was like a space age rocket ship! At one point I was driving around with $50K worth of Portmasters in the trunk of my car, hoping I didn't get rear-ended. They were not at all cheap, but they were worth it.
Yeah, friend. I’m very familiar, and it was amazing tech. It sure kept the data center cooling system busy, though.
Why use real physical modems when you already have subscribers signals converted to convenient digital form in DS1 bundle? Wouldnt it make more sense to put a box with one fat DSP doing 24 modems all in bulk inside a box with DS1 and Ethernet sockets at the ISP location instead?