The telecommunications market is expanding rapidly and becoming more substitutive and competitive. In this environment, demand forecasting is very difficult, yet important for both practitioners and researchers. In this paper, we adopt the modeling approach proposed by Jun and Park [Technological Forecasting and Social Change 61 (1999)]. The basic premise is that demand patterns result from choice behavior, where customers choose a product to maximize their utility. We apply a choice-based substitutive diffusion model to the Korean mobile telecommunication service market where digital service has completely replaced analog service. A choice-based competitive diffusion model is also formulated and applied on the case where two digital services compete. In comparison with Bass-type models, these two models provide superior fitting and forecasting performance. Finally, we suggest a new choice-based diffusion model to describe an environment in which substitution and competition occur simultaneously and show the application results. The choice-based model is useful in that it enables the description of such complicated environments and provides the flexibility to include marketing mix variables such as price and advertising in the regression analysis.