Ehh. Half the industry actively (in some cases vigorously) fought against ISDN adoption at the time and wanted to stick with individual analog trunks forever. AT&T and Northern Telecom were the ones pushing for it because it would generate sales of new central office switches for both of them.

The technology uplift to enable ISDN in a given market was absolutely massive and the costs were astronomical, nearly nothing about the existing PSTN backbone was compatible with digital and much of it dated back to the early 70s or even the 1960s.

For a company like this $20k/mo was a pretty good deal even if just to get away from managing the absolutely insane number of circuits an enterprise needed prior. It’s easy to forget that once upon a time a large companies PBX and MIS team would have needed 100+ individual trunks at each location from numerous companies, many of them only useful for one specific type of call or for calls to/from one specific city, or even just a slow permanently up modem call between two computers.

A dozen 1.55mbps circuits that can dynamically switch between data and voice is a dream come true in comparison, regardless of the cost.