I don’t know voice. I work on the data end of things, and there are so many solutions for moving more data faster and cheaper, that I assumed that ISDN went out the way 300 baud Hayes modems went out (faster modems using POTS are still in use at remote ATMs, POS, etc.).
Shows how little I know about a big topic.
Before the advent of MPLS circuits and SIP voice calls, PRI ISDN circuits were basically your only solution.
A simple PRI circuit can handle 23 simultaneous calls at one time, I’ve worked on sites that had a dozen or more of ISDN PRI circuits.
I was part of a team that supported the Voice Network for a worldwide network for a really large company, they had hundreds of ISDN PRI circuits, from telco providers from all around the world, basically every country had their own nationalized telco and they all supported ISDN PRI circuits for voice called.
You would be amazed at some of the most remote locations in the world that had ISDN PRI circuits installed.