If Nettalk is giving a list of their own routers and you don't have a freedom to choose your own and perhaps buy a new router with all extra great features, I would consider looking for another VOIP provider.
Honestly, VOIP should work with all different types of routers. I have been with a couple of different Providers and never had any issues with VOIP. Just a matter of opening up a port if needed. With the currect VOIP that we have-Ooma, I didn't have to do any port-forwarding in our Netgear Router.
Have a read at this Article
for some great VOIPs.
As for DMZ feature, you will find this on all modern routers.
Here's one great router
that I can recommend you. Always buy from a reputable Vendor for a hassle free return just in case you encountered issues.