In my smaller regional ISP we started off using USR courier and sportster modems with Cyclades cards before moving over to Livingston Port masters. I ended up selling the company before moving to all digital incoming lines (needed to support 56k modems).
Did exactly the same.... We had sporters hanging off the wall for a while (couldn't pack them tightly due to the crappy ventilation on the Sporters, but they were light, so clamping the cables to a suitable plate and fitting it to the wall worked great....)
The local telco had problems supplying enough lines from the nearest exchange, so they ended up hanging a thick cable bundle in the trees for several hundred meters, make a hole in one of our windows and put a large multiplexer cabinet in our office... Then three months later we moved to different offices - they were not pleased.
We got metal wire shelves and kept each shelf fairly close together (about 6-8 inches). We had 10 selves per rack and 18 modems per 3ft x 6ft shelf. 10 racks and related additional hardware (switches, terminals, routers, CSU/DSUs, etc) fit in about 600sq ft of office space with space to spare for technician workbenches. Could do much higher density these days of course, but it was the mid-90s and we thought we were doing good at the time.
We didn't quite get to the size where we needed that before we sold off the dial up business (for a pittance; we contributed to accidentally set off a price war in our market with incumbents with deep pockets - it was no fun, but I learned a lot)
I got lucky in that regard. The larger company in our area decided it was better to buy us out than compete on quality of service, so we were in a position to better negotiate terms of sale.
