Before ADSL was available and affordable, I did have ISDN at my place. It was charged by the minute, so I could not leave the data always on. I set up an always on Linux box at my apartment with a PCI ISDN adapter. A voice mail application answered calls on one of the 4 ISDN phone numbers.
I modified this voice mail application to accept a PIN code to open PPP on the other B-channel to the ISP and read back the dynamic assigned IP on the phone. This was something I could not do with a single analog phone line and a modem.
This was not the reason to get the ISDN, but this was the first time I could remotely access my systems at home at will. Not that I needed to that much, but I could.