Hi,
I have my VoIP box in between my router and my cable modem. Inside the VoIP box settings I set up a DMZ for the 655-router's reserved IP as handed out by the VoIP box. Inside the 655-Router setup I set it as STATIC IP using the IP from the VoIP box.
In this manner, I "believe" I avoid a double-NAT issue that some places say is a performance killing thing; not that I have seen it kill any performance either way.
SD1
If the WAN of your DIR-655 has a different IP than what you get when you visit http://whatismyip.com (a private IP on your DIR's WAN is a dead give away) then your double NAT'ed, it is that simple. Private IPs start with 10.x.x.x, 127.x.x.x, 172.16.x.x, 168.254.x.x or 192.168.x.x.
Double NAT'ing does not kill performance (other than adding a little latency so your traffic can be NAT'ed twice), it potentially kills specific types of traffic, and if configured wrong potentially all traffic. The biggest reason we preach against it though is it makes troubleshooting neigh impossible.
The real reason I am here though is to say it looks like your wandering off topic, you might try starting a new thread, or explaining the connection so that your problem gets the attention it deserves.
*** Modified by Fatman who forgot the original reason for his post.