It's actually relatively old. "Soft Dial Tone" (also known as "Quick Dial Tone") was trialed by most of the RBOCs back in the early 1990s (I think that it was a Telcordia/Bell Labs brainstorm), but was mostly abandoned due to the need for dedicated facilities (a completely connected line for every address served) and TNs (SDT/QDT is essentially a non-billed, restrictive voice type of service). It also proved very cumbersome to maintain, usually requiring two service orders (one for the customer, one for the SDT/QDT service) for every new connect and disconnect. The historical telco provisioning models don't have lines available for every possible address, since not all of them will have paying customers 100% of the time, so they play the percentages and only have lines available for the normal load of customers. SDT/QDT requires 100% connected lines all the time, and a TN assigned to every one of them, with a large percentage that will be non-paying facilities that still have to be maintained as if there were someone paying for it.
In the SBC territories that I'm familiar with, the Midwest region (nee' Ameritech) it was completely abandoned, and is not currently offered. In the Southwest region (nee' Southwestern Bell), it was also mostly abandoned, except for Texas, where there is a similar product known as Service Ready Drop is sold as part of the SmartMoves package for rental property owners and developers. However, when a live customer disconnects, it is not automatically re-installed, the way SDT/QDT is intended to work - the SmartMoves customer has to call in to have it re- established. It is also not available in SBC territory in Nevada (nee' Nevada Bell).
The one exception is California (nee Pacific Bell/Pacific Telesys). The California PUC has not only mandated QDT, it's actually written into CA law. QDT must be provisioned at all residential addresses "where technologies and facilities permit" (meaning that it's not currently available everywhere), and it is a mostly automated process of moving from QDT to customer voice and back again. As with most things the PUC's definition of "where technologies and facilities permit" (which includes TN availability as well as central office and outside line configurations) and SBC's tend to be very different, and occassionally causes some disputes about whether it can be offered in a given area or not.
I'd love to hear what the other RBOCs (Verizon, BellSouth, Qwest/USWest) experiences with SDT/QDT were/have been, as well as if any of the independents have experimented with it or are currently offering it.
Michael Chance