What about PBXs?

AFAIK, many PBXs still use ISDN to carry their incoming call data, or maybe this is some other sort of ISDN? From what I hear, most high-street retailers still use legacy systems using ISDN for stuff too. I would imagine BT are more making noise about this, just puffing up a bit and saying 'we're gunna kill this' to make people move now. I doubt they'll kill it for another few years yet though.