Registration Success on some networks, Fails on others

I am able to make and receive calls from two known WiFi networks with no problem. The phone automatically registered itself on both networks with no fuss. However, when I took the phone to a third (known) location, the registration failed. Unfortunately, the phone doesn't give me any information regarding the nature of the failure, so I'm having a hard time debugging. Is there some way I can see login attempts at pbxes.org?

The two networks that worked were simple home WiFi networks with wireless routers...one was an Apple Airport base station and the other is an Apple Airport Express connected to a wired Netgear router. One was completely open (no WEP/WPA) and the other uses WPA security. The network that doesn't work is a linksys base station using WEP security, but the phone is perfectly happy with it for surfing/email/IM on it. The base station is behind a more robust Cisco router at work, but I can confirm that no ports are being blocked. Any suggestions on how to debug the problem?

Make sure that third router is not one of the NetGear routers with SIP ports, or thatr 5060 is not forwarded to another device o the network, or that nothing is in the DMZ, or try changing the local SIP port on the WiFi Phone.

Pbxes.com is not port specific. You can use just about any port to make a connection. I deliberately set my device to a different port to reduce the number of conflicts that I would run into.