The same problem with the Linksys SPA-2102 failing to register cropped up again today. Despite switching from port 5060 to 5061 last week, it stopped working again today.

Switching up to port 5062 solved the problem immediately. I didn’t try switching down to 5060 to see if that was working again, although maybe I should have. I think my VOIP provider listens on all ports between 5060 and 5080, so if they’re not being reopened I’ll have a little while (at one port per week) before I’m in real trouble, but it will eventually become an issue.

I’ve been thinking about ways to test and see whether Comcast really is interfering with outgoing packets on certain ports, but haven’t come up with anything that seems like a really good test. The Java-based SIP test pages, of which there are many versions around, don’t seem to be cutting it. I could run netcat on a remote host somewhere, but unfortunately, most of the computers I have access to are on Comcast’s own network — so it wouldn’t tell me anything if Comcast is performing filtering at their upstream gateway.

hping seems like it might also be of some use, although I haven’t a clue about how to use it.