Could you tell me what firmware version this is as i have the same issue and would like to request a fix from draytek.

Cheers

Im running 'beta_0414' (see the post above yours ) supplied from Draytek Support.

I may have a related issue - I have a mediaserver on my home network on a fixed IP and it disappears every few days, rebooting the server makes no difference, rebooting the 2830 brings it back into view.

Just fitted a 2850 in my office and having the same issue.
I have three polycom IP330 phones which loose connection - "url call is disabled"
Rebooting the phone sometimes resolves the issue but only for a short period and find the router reboot lasts the longest.
This wasnt an issue on the draytek 2820 with 3.3.5.2_232201 only the 2830 & 2850 what has changed?

This is caused by the firmware shitting itself because it can't handle the quantity of UDP traffic so it overflows, I've seen this on many draytek and other brand routers.. It also happens with Bittorrent and other VPN's which use UDP (IP encapsulated over UDP)..

i had no problem setting up the viop phones cisco and a giga set phone i just tested my voip cisco phone as its like registering after phone boot up i had to either add a call filter rule for phone ie allow 192.168.2.255 any traffic and it worked no problem or disable the call filter and my skype issue tured off data filter