It was a Service, appears to still be, though likely no longer actively sold. It's one of the first digital services tariffed by the telephone companies (started in 1973.) Started as "Digital Data Service" but was renamed by AT&T as "Dataphone Digital Service". Both voice and data transmission are serviced at speeds from 2.4 to 56Kbps. This was pre-divestiture, so service and equipment were "one", I recall in the early 80s having a "modem" that had Dataphone on the front and the Bell System logo, but it wasn't an analog device -- or didn't work with POTS lines.

On 8/6/19 4:26 PM, Jeff G via vcf-midatlantic wrote:

Data-Phone (Dataphone) were a series of product that allowed devices (often computers but I don't know it if was limited to computers) to communicate across the telephone network. The earlier versions are not the familiar modems we had in the BBS error but were built for channelized T1, switched 56 and other AT&T services. I don't recall any service with the name Dataphone but I didn't do AT&T data services until the late 1980's/1990s. I recall seeing X25 switches and various DSUs with Dataphone. In the 80's I also seeing Dataphone on some Paradyne modems.