A mobile communication device with a wireless module and a controller module is provided. The wireless module performs wireless transceiving to and from a first service network and a second service network. The controller module transmits a first request message for a Mobility Management (MM) procedure to the first service network, and receives a first rejection message with an MM back-off timer corresponding to the first request message from the first service network via the wireless module. Also, the controller module starts the MM back-off timer for the first service network in response to the first rejection message, and transmits a second request message for the MM procedure to the second service network via the wireless module.