If your device supports ZRTP and the your end user's device supports ZRTP your call is private.
We support ZRTP by default, which is SRTP based on all calls, but instead of using SIP to exchange keys, we exchange keys within the media stream itself making it more secure. Each call automatically negotiates secure connections with endpoints that support the protocol. Currently, ZRTP is integrated into a few softphones, and is also available as an intermediary client application that intercepts your softphone's media and negotiates key exchange with any ZRTP enabled endpoint.
See Phone Apps for more...