Just a tip for future posts -- posting a problem like the one you've mentioned above without including a snippet from the logs or sngrep SIP flow is going to yield you little to no real help. There are literally hundreds of reasons why a phone may not be able to dial out... including logs is the only real way to isolate the problem so someone can help.
That being said, I have run into issues several times where a Grandstream device will register just fine but cannot dial out and switching the transport on the device from UDP to TCP or vice versa fixes it. I've never understood WHY that fixes it which bothers me to no end.. because at one customer site I can have Grandstreams that will only work using UDP, and on another site I can have Grandstreams that will only work using TCP.
Beats me. This is why I prefer Yealink.
