Call centers have relied historically, on Erlang-C based estimation formulas to help determine number of agent positions and queue parameters. These estimators have worked fairly well in traditional call centers, however recent trends such as skill-based routing, electronic channels and interactive call handling demand more sophisticated techniques. Discrete event simulation provides the necessary techniques to gain insight into these new trends, and helping to shape their current and future designs. This paper relates the experiences of designing call center simulations in Bell Canada. With the experience of constructing, executing and analysing a large call center model, problems that we faced are identified and potential solutions are given. The examples are taken from large and small call centers alike in the attempt to bring forth some common problems that a simulationist will face.