Recent studies have shown that mobile applications are increasingly becoming «chatty», resulting in excessive signaling load on cellular networks, potentially causing major network outages. One of the main causes of this signaling overload is the static dormancy timer that 3GPP-based networks employ to control the release of radio resources allocated to a device. To systematically manage the ever-increasing signaling load, we design a novel network-aware system named REACT for dynamically tuning the dormancy timers. In doing so, we highlight a key trade-off between signaling overhead on the network and energy consumed on mobile devices. Contrary to the prior work, our solution models and predicts traffic of mobile apps at the network side with high degree of accuracy, and then dynamically tunes the dormancy timer at a per-device, per-base-station or per-RNC granularity, the approach requires no modifications on mobile devices. Our evaluation shows that REACT reduces signaling overhead by more than a factor of 2 compared to current static setting of dormancy timers. Furthermore, network-aware dynamic tuning of dormancy timers enables a flexible system that can aggressively save energy on devices when the overall network load is low, and manages signaling overhead effectively during high network load.