sentences: Call Centers are important channels of communication within the consumer relationship and a point of integration between suppliers and their customers. Correctly sizing the capacity of a given Call Center can bring benefits not only in terms of improved customer service (efficacy), but also in terms of reduced operating costs (efficiency). However, specifying the capacity of a Call Center is not a trivial task, but one that demands a significant knowledge of mathematics, in particular of analytical models. This paper presents the Erlang B, Erlang C and Simulation models followed by a comparison based on a case study, in order to identify the advantages of using simulation.