The call center is an interesting area of application for queue models. In the call center queuing model, the clients are the callers and the servers are the calling agents. A call center, to operate effectively, must strike a balance between service level and service costs. The quality of the service provided is of primary importance. Therefore, the call center must at any time guarantee an appropriate number of servers with appropriate multi-skills according to the expected level of demand. This problem is commonly called the staffing problem. Thus, the paper aims to explain how to apply the waiting list model method to evaluate the performance of the multi-skilled call center indicator and to calculate the service level equation by introducing an optimization of the staffing problem for the optimal number of agents in each group. Finally, we used the results obtained through a numerical example to calculate steady-state probabilities, service levels, the optimal number of agents in each group, and performance metrics, and how these factors affect the entire system.