Here are descriptions of the more relevant changes in 15.0
This version carries a User Interface redesign of clients and server, plus many new devices and OS version support.
Despite SVoIP (Secure VoIP) protocols used, some PBX do not grant security, eg: => => SIP PBX => GSM device in clear. This call is a crypto-to-clear scenario and there is no way to notify the unsecure call to the caller. These PBXes can now be marked as untrusted: before bridging the crypto-to-clear call, both participants will hear an audio security disclaimer warning them that the call could be not secure.