The VoIP adapter has a NAT layer, aka firewall. This blocks any incoming unsolicited request. There's a downside to this protection, you can't forward ports from the outside. 
When I had my VoIP adapter in front of the router, I configured the VoIP to put my router in the DMZ of it's NAT layer so all the requests came through. This allowed my to basically ignore the VoIP box.
When I had my VoIP adapter in front of the router, I configured the VoIP to put my router in the DMZ of it's NAT layer so all the requests came through. This allowed my to basically ignore the VoIP box.