Telecommunication technologies show great changes all over the world. Next generation technologies replace older ones. Alternatives of the technologies force the users to make a choice between the competing technologies. Generally, an adverse effect is shown on the usage of older technologies when a new competitor is introduced to the market.In this paper, substitution effect on telecommunication technologies in Turkey is examined from the perspective of fixed-mobile substitution and next generation mobile technologies. The study uses two different technology diffusion models for each case: Logistic Substitution models and Gompertz model. Generic diffusion model and substitution model are compared in terms of RMSE and MAD. Fixed-line and mobile telecommunication technologies are inspected together to see substitution effect of mobile telecommunication on fixed-line, firstly. In the second step, fast technological change in the mobile telecommunication technology is handled for Turkey. 2G, 3G and 4.5 G technologies are analyzed to see diffusion and substitution process of these technologies. All the results indicate that; Logistic Substitution model is better to simulate the systems in competitive environments. On the other hand, 2G technology is found the most affected technology by the substitution.