I spent the last few days trying to figure out why kphone wouldn't work with my newly aquired Belkin router. The clue was the following security log:
15.11.2004 19:36:52 **UDP Flood to Host** 192.168.2.3, 32860->>
The problem is that the router thinks that SIP incoming packets are a
denial of service attack. You'll need to follow this
undocumented link. Next turn off SPI and Anti-DoS firewall protection.
I find it most uncool that Belkin did not document this link. It would have saved me a lot time had they mention it on their manual.