Proton therapy is a highly targeted radiation treatment for tumors. In a typical proton therapy system, multiple gantries share a beam accelerator. A patient is often called back to a gantry immediately after the gantry becomes available, and each treatment requires multiple beams. This causes gantries to compete for the beam and results in long beam wait time, which negatively impacts treatment quality and patient satisfaction. In this study, we propose a rollout-based gantry call-back control method, considering both beam wait time and beam utilization. When a gantry becomes available, an optimal call-back delay time is applied to mitigate beam request conflicts. Simulation experiments suggest the proposed method can be used to find the trade-off between beam wait time and the bean utilization and improve proton therapy care delivery process.