That whole business sounds very strange to me, even if it's on Twitter's radar.
According to the sources, others are involved, too, including Facebook.

Sources tell us that Twitter is in negotiations to buy ZipDial, a startup founded in India that has honed in on a mobile use case that is unique to its home market and others like it. People who call numbers but hang up before the call is answered, using the action as a simple alert that doesn’t cost any money. That alert, in turn, becomes the basis for a number of other, commercial actions, and analytics to track everything.Report: Twitter In Talks To Acquire India's ZipDial [techcrunch.com]