[fenesiistvan]

With regular SIP you might have a look at mizutech SIP encryption. It has multiple encryption methods (the standard TLS/SRTP and non standard RSA based) and also a nice obfuscation to bypass VoIP blockages. They are also running a distributed network to mask the VoIP servers. http://www.mizu-voip.com/Software/VoIPTunnel.aspx