A User Equipment (UE) is provided. The UE includes a wireless transceiver and a controller. The wireless transceiver is configured to perform wireless transmission and reception to and from a mobile communication network. The controller is configured to accumulate Transport Block Size (TBS) assigned by the mobile communication network to obtain an estimated TBS during each first period, and obtain an estimated bitrate of a plurality of voice frames to be transmitted to the mobile communication network according to the estimated TBS. When the estimated bitrate of voice frames is different from current bitrate of the voice frames during a second period, the controller is configured to adjust the current bitrate according to the estimated bitrate.