We study a call center service system with some extension lines.The customer arrivals follow a Markovian arrival process(MAP).A customer first accepts a machine service by choosing an option from the voice menu after arrival.Customers leave the system if they are satisfied by the machine answer,otherwise,they continue to accept an agent service.A customer may need to wait in the line if all agents are occupied by other customers.Both the machine service time and the agent service time are exponential distributed.construct the transition matrix,then derive the recursive equations to calculate stationary probability distribution.The numerical analysis shows how to obtain the optimal number of staff to maintain the given service level.