One of the critical problems in the call center industries is the staffing problem since they must face variable demands and because staff costs represent a major part of the costs of this industry. The problem of dimensioning a call center is modeled as a particular scheduling problem, where the objective is to minimize the weighted number of resources needed to perform the jobs. We present in this paper a first constraint programming approach and some improvements, to solve this problem. We show that the results are very sensitive to the constraints definition and to the instanciation methods. Models are tested and results are discussed.