A telephone switchboard provides various operating voltages to a telephone in different operation modes. The telephone switchboard includes a control unit, an output control circuit and a voltage converter. The control unit detects the operation mode of a telephone and generates control signals to the output control unit. The output control unit controls direction of current flowing through the telephone according to the control signal, and generates a feedback signal according to any change in the operation mode of the telephone. The voltage converter receives the feedback signal and converts the received power to a suitable output operation voltage according to the feedback signal. The telephone switchboard is capable of providing various operation voltages to the telephone in the different operation modes.