A dual network telephone device selectively performs a telephone communication via a public telephone network and via an IP telephone network. The dual network telephone device generates IP telephone identification information for a non-IP telephone device registers to an IP telephone server an association of an IP address of the dual network telephone device and the IP telephone identification information for the non-IP telephone device, receives a first call request sent from the IP telephone server, sends a call request notice instruction to the non-IP telephone device when the first call request includes the IP telephone identification information for the non-IP telephone device as a call request destination, receives IP telephone identification information input to and sent from the non-IP telephone device, sends a second call request to the IP telephone server.