What do you mean specifically by ‘access’? Are you talking about logging into the Phone GUI? If so, it can be done but whether it’s simple or not will depend. You can establish an ssh session to the PBX, then configure a SOCKS tunnel via the ssh session, and then configure your browser to proxy http thru the tunnel and then login to the phone using the VPN IP. Easy peasy. You might also be able to establish a VPN connection to the PBX from your PC and route traffic to the other VPN IPs thru this connection.