A voice over Long Term Evolution (VoLTE) capable mobile device is configured to deregister from an Internet Protocol (IP) Multimedia Subsystem (IMS) network element for voice connections and use a dual network mode of operation that includes data connections via a 4G LTE/LTE-A network and voice connections via a legacy 3G or 2G network based on performance metrics a serving cell of the 4G LTE/LTE-A network. When the performance metrics indicate that voice connections may be unstable, such as when a reference signal received power (RSRP) falls below a threshold level but remains above a reselection/handover threshold level, the mobile device deregisters from the IMS network element to disable VoLTE connections from being established. The mobile device establishes the requested voice connection via a legacy 3G or 2G network instead, while established or new data connections for the mobile device continue to use the 4G LTE/LTE-A network.