I can relate what I recently had to do to get my VoIP ATA to work through my DIR-655:
1. In the Advanced - Firewall Settings page, make sure SIP is unchecked in the ALG settings (last section on the page). Then, try again.
My case was a little different, here's what I had to do:
It ended up that my ATA's WAN address was the same as my DIR-655's LAN address. I changed my ATAs WAN IP address to 192.168.3.1 to avoid this conflict (this was done by logging into the ATA's web interface)
I then had to put the ATA in my router's DMZ, and had to give it a LAN IP address outside of my Router's LAN DHCP scope. Since my LAN DHCP scope ends at 192.168.2.199, I gave the ATA device LAN IP of 192.168.2.200 (again, done by logging into the ATA's web interface).
Since I've done this, my VoIP ATA works great behind the DIR-655.