A mobile communication device including a wireless transceiver and a wireless transceiver is provided. The wireless transceiver performs wireless transmission and reception to and from a first service network and a second service network. The controller uses a first subscriber identity card to establish one or more first radio bearers with the first service network via the wireless transceiver, uses a second subscriber identity card to set up an Internet Protocol Security (IPsec) tunnel with an evolved Packet Data Gateway (ePDG) of the second service network via the first radio bearers, and uses the second subscriber identity card to register an IP Multimedia Subsystem (IMS) service with the second service network via the IPsec tunnel.