ISDN is actually used quite a bit in niche markets in the states. For studios, typically you'll see a couple B-channels bonded to give someone 128K MP3 or AAC. At a bare minimum, 64 kbps/16khz g.722 is used. I worked in a newsroom a few years ago where I frequently had to record a weather guy from a distant area who used this.

Used in STE units for secure communication as well:

https://en.wikipedia.org/wiki/Secure_Terminal_Equipment

I think it sounded good because it was clean 64K end-to-end. No trickery with transcoding a bunch of times along the way so someone could save a nanopenny.

Right. The bits you send are the bits that are received, clocked at a fixed rate. No compression or timing artifacts.