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How it works
DSVD allows users of two DSVD capable modems to simultaneously engage in a point-to-point voice conversation and data transmission. DSVD does not appear to allow dialup internet users or users of point to point modem connections (such as to the office or a Bulletin Board System) to make or receive arbitrary voice calls while online.
In ordinary circumstances though this did not work. DSVD only works when the user's internet service provider and telephone service provider are the same organization. On top of this regulatory requirement special equipment is required at the telco is required at the Subscriber Line Interface Circuit (SLIC).
V.70 vs VOIP & ISDN
Today, such a service, if offered, would typically be provided via VoIP, ISDN, and/or via DSL over the same wires as an analog POTS line. This standard was released in 1996, at a time when consumer access to The Internet was beginning to take off as were Cell Phones.
DSVD was not substantially adopted, let alone extended to provide the features consumers actually needed (like making arbitrary calls while connected). V.70 was largely obsoleted by the services offered and the changes in usage that accompanied the rise of The Internet.
In the internet model, the user's modem does not connect directly to a site where there may be parties the user wishes to have a voice conversation with but instead connects to an intermediate Network Access Point; as such, the small subset of potential phone conversations served by DSVD was further reduced.
When dialup Internet was the only way to access the Internet, the ability to carry out an arbitrary voice conversation while online would have been useful. To do so however required a significant and expensive infrastructure upgrade, standardization, and cooperation between ISPs and telcos. The solution to the problem would have been obsolete before it was deployed. DSVD offererings were so delayed that VOIP for all practical purposes displaced it.
DSVD also does not define a standard for carrying voice and data traffic simultaneously over the interface between a computer and a modem, something that could still be useful today over a USB connection to a cell phone modem, so it is not dead as a technology.