Sizing the capacity of a call center is a well known problem. However, in some cases there are regulatory aspects that must be observed that, combined with specificities of the service provided and how it operates, affects the manner of the process for forecasting the inbound call rate and decisions on sizing capacity. This paper presents a real problem relating to several electric power distributors in Brazil regulated by ANEEL (the Brazilian regulatory agency for electric power companies). Amongst the available data, this kind of call center is affected by commercial actions, debt collection actions, billing activities, the health of the state of the distribution network and maintenance activities, which are not integrated and planned together. Considering the dynamics on the company cash flow these factors may change and adversely affect the demands on the call center. Thus in this article, a description is given of how experts' knowledge has been integrated so as to improve the forecast of incoming calls and the decision process. For this purpose a model to evaluate the sizing of this type of call center is developed in order to comply with regulatory requirements for this service.