The purpose of this paper is to examine whether there are relationships of substitutability or complementarity between the call demand of fixed telecommunication and mobile telecommunication services. In order to clarify the relationships, we suggest a model of call demand of fixed telecommunication service which incorporates the variables representing the diffusion of mobile telecommunications subscription and estimate it with relevant time series data. Finally it is shown that there is complementary relationship between the call demand of fixed telecommunication and mobile paging services, and it is also found that there is substitutable relationship between the long distance call demand of fixed telecommunication and mobile phone services.