VoIP Phones - Draytek
Draytek 2850/2930 and probably others
Problems with registration?
These routers seem to have very short a UDP NAT expiry ~30sec? The port was closing before the phone re-registered, causing problems with incoming calls not working (as the phone is no longer registered).
Changing the UDP timeout to 3600 on the router will help. Log in to the console, and use the command:
portmaptime -u 3600
Here's a screenshot of the web interface, the purple arrow highlights the button to open the command console through the web interface, and an example of the portmaptime command both to query the current UDP timeout, to set it to 3600, and then to confirm that it has been changed: