Consider Mumble as an openvpn/asterisk alternative #10
Description
Mumble, popular with online gamers is an encrypted voice chat application. I have not looked into how this would be integrated but is likely much more lightweight then openvpn/asterisk. An added benefit is that there is a mobile version of the client, and while there is Openvpn and sip clients for devices you need a hacked android to get openvpn working and I dont think you can get it working at all on iPhone.