Is LAN one of the options for VoIP bound interface? If you are having trouble configuring the WAN port to be untagged one way could be to bypass the WAN port just connect the ERL to one of the NF4V LAN ports and make sure LAN IP/DNS settings are configured.
You need to check whether the NF4V itself has internet access, it doesn't look like there is a ping facility in the web GUI but there should be through TELNET. Or you could check the route table has a path to the internet under the Device Info menu.










