Technically I don’t see why you couldn’t run OpenVPN server on the same box but not sure I would recommend it. Is there a reason you want to set it up this way? I would personal have a firewall running OpenVPN in front of the FusionPBX. We use pfsense for this which works well. That would be my recommendation.
If you still want to run OpenVPN on the FusionPBX box, then simple follow instruction for installing OpenVPN on the Linux disto you are using.
the requirement is to encrypt the call signalling and audio (rtp packets). I was thinking of using openvpn for secured call connection.
hence the setup should be
IP Phone <-> openvpn server <-> Fusion PBX
^
IP Phone <-----|
I would rephrase my question as How can Fusion PBX connect to Openvpn Server ? Does Fusion PBX support Openvpn client ?
I have deployed Wireguard server on fusionPBX box and works fine except some MTU issue was there and cant use UDP for phones and TCP is used. Client side was a DD-WRT router.