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OpenVPN will start and run as a icon tray icon (bottom right hand corner of your screen). Find this icon, right click on it and choose "Connect".
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Now repalce extn999.key with the actual name of your key file.
Configure Softphone
You need to reconfigure the softphones account so the domain uses the openvpn servers ip address.
If you have used all of the default settings when installing the openvpn server then the requried ip address will be:
192.168.99.1