ZRTP
From wikipedia:
ZRTP is a cryptographic key-agreement protocol to negotiate the keys for encryption between two end points in a Voice over Internet Protocol (VoIP) phone telephony call based on the Real-time Transport Protocol. It uses Diffie-Hellman key exchange and the Secure Real-time Transport Protocol (SRTP) for encryption. ZRTP was developed by Phil Zimmermann, with help from Zooko Wilcox-O'Hearn and Colin Plumb and was submitted to the Internet Engineering Task Force (IETF) by Phil Zimmermann, Jon Callas and Alan Johnston on March 5, 2006 [1] and published on April 11, 2011 as RFC 6189
It's a candidate for use in the GlobalNettizensTelephoneSystem