Most likely you are trying to work behind a router doing NAT.
The voice packets find the way out, from the NAT to the WAN, because the only information needed is the “default gateway” in order to go from the LAN to the WAN.
But the packets from the WAN to the LAN are not making it trought, either because you are not on the DMZ of the router, or you do not have the proper ports forwarded from WAN to LAN.
Try the easy solutions first: Put the Quintum on the DMZ of the router.
If that still does not work, you may need to setup the router to do port forwarding , the Quintum site have a document listing the ports that you need to setup.