Both voice and data are indispensable services in current cellular networks. In this work, we study the inter-play of voice and data in operational LTE networks. We assess how the popular CSFBbased voice service affects the IP-based data sessions in 4G LTE networks, and visa versa. Our findings reveal that the interference between them is mutual. On one hand, voice calls may incur throughput drop, lost 4G connectivity, and application aborts for data sessions. One the other hand, users may miss incoming voice calls when turning on data access. The fundamental problem is that, signaling and control for circuit-switched voice and packetswitched data have dependency and coupling effect via the LTE phone client. We further propose fixes to the identified issues.