Cisco 7960 registration issues behind Linksys router NAT

I've been using a Cisco 7960 in my home office interacting with a dedicated server I have setup with FreePBX (I do realize this is a Trixbox forum), with my consumer-grade D-Link router. It just works.

I bought some additional 7960s for a client of mine who was to share the same asterisk server, and configured them behind my D-Link to test them. After shipping them and getting them connected at my client's office, who was using a Linksys WRT54g (with DDWRT firmware), they would NOT register at all with Trixbox. The IP address of the phones as shown on the asterisk CLI (sip show channels) was showing up as a private IP (192.168.0.xxx), so I knew it was a NAT issue, however these phones had worked perfectly from my office just days before. As a test, I took my 7960 to another client's office, and it failed to work behind another WRT54g with stock firmware.

When I started getting desperate, I suggested the client buy a new router. They did (another Linksys, the WRT160n), and the same registration/NAT issue was there. By this point, I was frantic. These are expensive phones, and would be total bricks if for some reason we could not get them to talk to asterisk properly. So I mailed the client my D-Link WBR-2310 and bought myself a new D-Link DIR-628 (does G or N, good price). The new one worked perfectly, and the client is now using my old one and it works PERFECTLY.

So, for all those having NAT issues with Cisco phones and Linksys routers, do give D-Link routers a shot if you are able to. The fact that Cisco == Linksys doesn't seem to matter, because these phones just wouldn't work for me behind their NAT.