A telephone apparatus and method for stabilizing a call in a no power operation (NPO) mode. A sub-controller controls the NPO mode by at least one of a central office line voltage and an external supply voltage, and a main controller performs a booting operation in response to the external supply voltage to control an operation of the sub-controller under the condition that the external supply voltage is provided. A power-on detector detects a power-on state to output a power-on detection signal to the sub-controller, whereupon, during a call connection in the NPO mode, the sub-controller outputs a booting hold or reset signal to the main controller to prevent the main controller from booting a program masked in an internal ROM area. A hook-on signal based on a hook-on state of a handset, causes the sub-controller to block the output of the hold or reset signal, enabling the main controller to perform the booting operation after a call termination occurs.