The Italian deregulation process of telecommunications market in the last years has produced a large economic impact since it has altered equilibriums that were established for a long time. In this framework, we notice a strong need for adequate tools to analyze the market and its trends and, at the same time, a lack of specific solutions within the scientific literature, due to the new technical challenges issued by the problem. In particular, in the context of building a Decision Support System (DSS) for the strategic marketing unit of TELECOM Italia (TI) we have devised a new methodology to profitably combine most powerful tools from KDD and Economic Sciences. We have tested our approach by analyzing the residential telecommunications market demand in Italy during the transition from a monopolistic structure to an oligopolistic one. In this paper, we first address the state of the art in DSS design, then we describe the proposed methodology and its application in the case study.