Mobile communication devices and integrated circuits thereof are provided. The mobile communication device, communicating with a packet switched network and a second network, includes a transceiver, a first PLL circuit and a controller. When exchanging an ongoing data transmission with the packet switched network, the transceiver transmits a first message to the packet switched network, which indicates the mobile communication device will be tuned away from the packet switched network, and performs a subsequent procedure for the second network. The first PLL circuit provides a first clock to the transceiver. When the ongoing data transmission with the packet switched network is not completed, the controller controls the first PLL circuit to switch the first clock from a first clock frequency to a second clock frequency after the transceiver transmits the first indication message.