Well, this tech is derived from the project which was designed to meet specs of one of our clients. We did propose ZRTP during design phase, to the client but they security analysts decided against it. They affirm that given the state the current state of art in speech recognition and synthesis ZRTP can be vulnerable on impersonation during short code validation phase for the attacker with sufficient resources. I'm personally doubtful, but one thing i'm sure about, is that this client security experts have access to info and resources which are not available to me.