Technical question re: incoming calls & Asterisk

[jdaskew]

I set up a new Asterisk system (PBX in a Flash) and am having some difficulty connecting via Voxalot. I suspect that I'm missing something and hope that someone has some advice or insight.

My system is currently registering to us.voxalot.com. Asterisk uses SRV records, so the registration ends up going to one of the proxies listed at _sip._udp.us.voxalot.com (there are four proxies listed). Looks like a round robin DNS, so the registration tends to move around throughout the day.

When calls come in, it seems that it can come from any of the four proxies - not necessarily the one I am currently registered to. If the call happens to come from the registered proxy, everything is fine. Asterisk associates the call with voxalot and sends it thorugh. If the call comes from one of the other proxies, Asterisk does not make the association and sends it to the anonymous (from-sip-external) context.

Now Asterisk could remedy this by comparing against all of the SRV records as opposed to just comparing against the currently registered one, but I believe there is another problem...

For any client (not just Asterisk) that is behind a NAT, I would think that incoming INVITES from the other proxies would get dropped. As I understand it, the only proxies that could send an invite would be ones that were recently contacted (for practical purposes, this would be the currently registered proxy).

Please let me know if I'm overlooking something (obvious or otherwise) or if this is a know issue.

Happy New Year!

Edit: I have "Sym NAT" turned on in my account.