VoIP Secure Communication Protocol satisfying Backward Compatibility

Several VoIP security protocols have been standardized for preventing many VoIP threats. But since several VoIP carriers have different VoIP protocols, end-to-end VoIP secure communication can not be possible. Also when initiator using secure VoIP phone wants a secure communication with responder using non-VoIP phone such as PSTN phone, or cellular phone, etc, so does it. Therefore, we propose VoIP secure communication protocols satisfying backward compatibility.