ToolBox -

The easiest way to secure voice traffic between locations is to use VPN Tunnels (IPSec) – this encrypts all data (including Voice) between two locations and is relatively easy to configure. This method works with standard VoIP software and hardware.

Our business VoIP systems have numerous options to secure VoIP without needing VPN tunnels. This is useful for locating IP Headsets outside of your network without requiring a VPN tunnel.

SRTP (Secure RTP) and SSL/TLS are both implemented by our PBX platform and our IP Telephones, this ensures the end-to-end security of your voice traffic.

Secure RTP
http://www.ietf.org/rfc/rfc3711.txt

SSL/TLS
www.ietf.org/rfc/rfc2246.txt