In Australia ISDN was available pretty much everywhere to any residential customer who wanted to pay for it, but it was not particularly popular. It was widely used by businesses, however.

The main reason was that the line rental was significantly higher, POTS lines had untimed flat rate local calls, whereas ISDN didn’t, and ISPs charged more for ISDN plans.

Towards the end of the 90s that changed for voice calls, and so a fair number of folks who couldn’t get ADSL for one reason or another got DoV (Data over Voice) Internet services. This is where you make a single channel ISDN “voice” call, and then use it for 64Kbps data. Most ISPs supported it on ordinary dialup plans.

This gave you significantly better speed, and much improved latency compared with what could realistically be achieved with V.90.