We've deployed quite a few Draytek 2820's over the last year or so along with Asterisk PBX's. 

What we found with a significant number of installs, is that every few days, Asterisk would report the termination servers as being unreachable.  We explored Asterisk at first, but we then found a reboot of the modem would clear the issue. 

We didn't have this issue with any Cisco Routers. We've reported this to Draytek support a number of times, been given beta firmware etc, no change.  It should be said that all other functions were fine like Internet browsing etc.  If we changed the server asterisk was pointing to, then that worked without a reboot.

 

We lived with auto reboots on a daily basis, but there were times during the day when the host became unreachable again.   It was a real shame, because otherwise these were great routers for the price.

By chance, we happened on a fix, which touch wood, appears to resolve the issue.  Within the firewall settings, general, ensure that 'Allow large fragmented UDP packets' is turned off.  The default is to have it on.

Hope this is of use for someone out there.