Go back and read it again. No where in that post did I tell him to setup port forwarding. In fact I told him to delete all of them. He is using pfSense as his router. I seriously doubt you are.

The talk about static port setup has nothing to do with your device. Its something related to our particular router setups. Not one single SOHO router I know of randomizes ports.

As I said-
The solution is for someone running a commercial style router/firewall where firewall rules can be set up separate from the port forwarding functions. But look at the reasoning.
What I am telling you is that your firewall/router combined with the way this particular voip works is what is causing your issues. I linked to the post so that you could see why your setup is not passing audio. But I told you here to contact support. I did not recommend any solution as I know nothing about your internet connection.

Im simply showing you that your statement above "When I call tech support they try to blame unrelated things like~ port forwarding." is incorrect. It is not unrelated and they are not blaming the port forwarding but instead using it as a tool to cause your particular firewall/router to pass the audio to your device. Since most homeowner type devices like you buy at Staples or Walmart are pretty un-user friendly when it comes to opening firewall rules then support uses the next best thing which is to get the device to open its firewall to the audio streams by using port forwarding. If you don't like the solution then give a little more information about your particular internet connection and which RTP ports your new VOIP device is set up to use and maybe someone can throw some other clues your direction.

Truthfully Im not a big fan of the Grandstreams either but most people including my accountants office seem to have real good luck with them and they help Voipo keep their prices where they are.