Technically speaking ISDN is a data service rather than voice, being the early digital solution before ADSL. It was for many years though the way of delivering multiple voice connections effectively and is still very popular in the world of radio broadcast. Its popularity stems from the fact it creates an end-to-end connection between, for example, the outside broadcast location and the studio meaning there is very low latency and high reliability. ISDN carries a significant premium in terms of cost above PSTN and is not quite so widely available.
The use of ISDN is falling rapidly due to its cost and the increasing confidence of broadcasters in using newer IP based solutions which run over existing internet connections.