Hi Var,
Sure, port forwarding (DNAT in Linux
iptables(8) terms) can be done in Access Server. But SIP behind NAT is tricky and not recommended. You need to configure routing and use the VPN address of the freepbx host in your VPN/SIP client.
See this page for information:
Global Routing in OpenVPN Access Server
Note that you just said "openvpn" in your post. Answers in this subforum will be specific to
OpenVPN Access Server, and not applicable to the open source version of openvpn.
regards, rob0