DMZ may still work. The card would not be any more vulnerable to hacking in the DMZ than it would with its own public ip.
With VoIP, sometimes a DMZ still won't work. I don't know much about Vertical but as one example Toshiba CIX IP phones have never worked through any NAT, even with DMZ. Sometimes I have to explain to a customer's IT that I really do need another public IP address (or a VPN) for their remote phones to work.
Toshiba finally got it to work with NAT on their IPEdge system by using a relay service that knows how to manage NAT IP information.