In 3G cellular networks, the release of radio resources is controlled by inactivity timers. However, the timeout value itself, also known as the tail time, can last up to 15 seconds due to the necessity of trading off resource utilization efficiency for low management overhead and good stability, thus wasting considerable amount of radio resources and battery energy at user handsets. In this paper, we propose Tail Optimization Protocol (TOP), which enables cooperation between the phone and the radio access network to eliminate the tail whenever possible. Intuitively, applications can often accurately predict a long idle time. Therefore the phone can notify the cellular network on such an imminent tail, allowing the latter to immediately release radio resources. To realize TOP, we utilize a recent proposal of 3GPP specification called fast dormancy, a mechanism for a handset to notify the cellular network for immediate radio resource release. TOP thus requires no change to the cellular infrastructure and only minimal changes to smartphone applications. Our experimental results based on real traces show that with a reasonable prediction accuracy, TOP saves the overall radio energy (up to 17%) and radio resources (up to 14%) by reducing tail times by up to 60%. For applications such as multimedia streaming, TOP can achieve even more significant savings of radio energy (up to 60%) and radio resources (up to 50%).