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Femtocell is an effective solution to improve indoor coverage for cellular networks, where short-range and low-power Base Stations (BSs) called Femto BSs are deployed in small areas (e.g., buildings). We propose the Femto Private Branch Exchange (FPBX) to concentrate the traffic of the Femto BSs in a specific area, such as a campus or an enterprise with central or distributed locations. Through the FPBX, a normal cellular call between two Femto users can be replaced by a low-cost extension call. Therefore, the call routing cost can be significantly reduced. In this approach, the existing mobile network nodes are not modified. We analytically analyze the call routing performance for the FPBX approach, and develop simulation experiments to validate against the analytic model. Our study indicates that the FPBX approach can effectively reduces the call setup costs and the voice trunk costs among Femto users by slightly increasing the normal cellular call setup costs.