Long-Term Evolution (LTE) is the latest development in wide area cellular mobile network technology and it is all-IP packet-switched network. Voice over IP is used to provide voice service to LTE users. The speech frames are encapsulated into RTP packets and sent over the network. The underlying UDP and IP layers prepend their headers to this small RTP packet resulting in a relatively high overhead. The small size of the RTP packets leads to an overhead problem, thus wasting network resources. This book proposes to multiplex RTP and data packets at the user’s device as a solution to reduce the overhead. Moreover, the capability of modern user devices to switch between several interfaces (such as LTE and WLAN), is taken into account and the multiplexing of multiple traffic flows or a single traffic flow are studied in the case of a vertical handover. Performance and cost metrics are used to evaluate different potential demultiplexing points. The increased packet payload size increases the energy efficiency of LTE. In addition, to ensure high quality of service for VoIP traffic, the simultaneous use of multiple interfaces is efficient if the multiplexer is enabled.