OK, I would think this is something that Skuzzy would know much more about, but here's my crazy thinking.
Incoming VOX must be coming in on a TCP or UDP port, and it wouldn't surprise me to learn that different arenas use different ports.
OK, now you say you've turned off all firewalling/security products, and even connected the PCs directly to your cablemodem, which is good troubleshooting data. I am ALSO assuming that you took the unusual (and normally un-recommended step) of turning off all the software security products and firewalls when the PC was connected directly to the cablemodem and you still couldn't get incoming VOX.
If all that is true, then I would only have the following conclusion: that your ISP is blocking certain incoming ports at their end, so your machine never even sees it. However, it isn't blocking ALL the different ports incoming VOX uses, which is why you can get incoming VOX in some arenas.
(This theory only works, BTW, if Skuzzy can confirm that different arenas use different ports for incoming VOX. In a perfect world, he would even tell us, right here, what those different ports are, specifically.)
Now I know this is crazy and probably unlikely, but all your other troubleshooting activities have ruled almost everything else.
So here are the tests I would do to prove this theory.
Test 1: Take your computer, or your kid's, and connect it to the internet via some other method. Take it to work. Take it to a friend's who has a different ISP than you. Heck, try a trial dialup account with a different ISP. See if VOX comes in in all the arenas. If it does, then your ISP is having a problem.
Test 2: find out what ports those arenas are using for VOX. Then set up a server (say, an FTP server) on your computer that is set to listen to those ports. OPen up your firewall/router for those ports. Now go to a different PC (maybe at work, or a friend's), and try to FTP into your server on that port. If it works, then your ISP is not to blame. If it fails, it's time to look hard at the ISP.
If it really is the ISP, then things get trickier. No tech support droid there is really empowered to open ports, or even check for them. Heck, some might not even know what you're talking about.
Anyway, these are the things I would check. If you do, please let us know what happens.
-Llama