A Voice over Internet protocol (VoIP) device includes a time division multiplexing (TDM) bus, a plurality of digital signal processors (DSPs), and a plurality of subscriber line interface circuits (SLICs). The SLICs are respectively connected to a corresponding plurality of telephones. The VoIP device distributes the TDM bus to a plurality of calling timeslots and a special timeslot, and allocates at least one of the calling timeslots to each of the SLICs, selecting one of the DSPs as a special DSP. The VoIP device further directs the special DSP to generate an alternating voltage signal to the special timeslot, and directs the SLICs to receive the alternating voltage signal from the special DSP by the special timeslot to prevent the VoIP device from being locked at a high voltage and from generating noise.