The communication system comprises a base station (BS), a femtocell and a core network. The femtocell deployed within the signal coverage of the BS establishes a first wired connection with an internet service provider (ISP) network and a first wireless connection with a user equipment (UE). The core network establishes a second wired connection with the ISP network and a third wired connection with the BS. The core network is configured to determine that a voice data transmission service is about to be provided to the UE, and transmit a first radio access bearer (RAB) setup request message to the BS to enable the BS to establish a second wireless connection with the femtocell according to the first RAB setup request message so that the UE performs a voice data transmission with the core network via the first wireless connection, the second wireless connection and the third wired connection.