These are the instructions to configure OpenVPN + SIP configuration, based on a brainstorming discussion of the Asterisk Users Mailing List.
The server is running on a uClinux appliance, with /dev/net/tun, and OpenVPN is 2.0.9. The clients will be Windows hosts connecting through Ethernet in hotels or public wifi hotspots.
1. Install OpenVPN on Asterisk server. On appliance, there’s only a single binary /bin/openvpn, and configuration files are in /etc/openvpn/.
To be positive SIP/RTP packets go through the OpenVPN tunnel, make sure the firewall in front of the OpenVPN/Asterisk server only has OpenVPN port open (default: UDP 1194).
2. On client, from http://www.openvpn.net, download and install OpenVPN for Windows, which includes Service + GUI