In recent years, mobile phone service has become the most popular term of telecommunication service. However, in back of the complex pricing plans and promotion plans, there are some interesting tropics. For example, how does a mobile phone service operator price the on-net and off-net charges? What is the factor that makes the off-net charges more expensively than the on-net one? And which principle is adopted to design the multiple pricing plans by the operators? In this study, we will make some amendment for the Hotelling Model, and set a new model which be called the Psychological Differential Model, that can expectably explain the eventuality which a man has a lot of phone number from different operators simultaneously. Besides, it will add a factor which will derogate the network externalities of the utility of mobile phone service. Mathematically, we use the way of ¡§Two-stage Market Pricing¡¨ to develop the equilibrium of on-net and off-net charges. By the cost pricing method, we can find the access fee will cause off-net charges more expensively than on-net one in the chapter three of this paper. Finally, in the chapter four, this study will introduce the principle of the third price discrimination to explain the outcome of multiple pricing plans.