In providing call backup of a VoIP terminal, where the VoIP terminal connected to a VoIP network is automatically provided with a PSTN backup function, the on the hook state is deferred for a predetermined time period even when the on the hook state is required to be performed while a user makes an urgent communication, and a ring tone signal transmitted from a PSTN can be detected even when the telephone connected to the VoIP terminal is in a VoIP mode state. Simultaneously, the telephone connected to the VoIP terminal senses the hook state of the telephone in the state of the PSTN, and can be automatically connected to the VoIP network upon the VoIP terminal being on the hook.