This paper studies the effects of cell residence times on wireless private branch exchange (WPBX) systems. In a WPBX, several base stations are connected to a PBX, and the PBX is connected to the Public Switched Telephone Network. It is important to determine the external line capacity and the radio channel capacities of a WPBX to optimize its system performance. Our previous study utilized an analytic model to study WPBX without user mobility. This paper proposes both the analytic and the simulation models to investigate the performance for WPBX with user mobility. Specifically, we study the effects of handoff and the variance of the cell residence times on resource planning. Based on the workload to the WPBX, our study provides several guidelines to determine the capacities for the PBX and the base stations.