A VoIP device digitises the voice conversation, chops it into packets, and sends the packets using the UDP protocol. If the VoIP device at the other end is using a packet size of 10ms, then you would be receiving 100 UDP packets/second just for one VoIP call.
Your DrayTek router is seeing these packets – plus other inward UDP packets from other applications – exceeding the threshold value, and so the router defends the “attack” by ignoring the incoming packets.
The quick answer is to increase the DoS Threshold values, or disable those particular defences.
But what are those other UDP packets? Are they legitimate and acceptable?
P2P file sharing applications typically connect to hundreds of other hosts and can congest your bandwidth. If you are running any P2P application, turn it off for a while and see if the DoS Defence still triggers (and the router still reboots). Since P2P programs generate quite a lot of outgoing traffic (even if no files are being uploaded from you), we strongly recommend that you check your P2P program’s configuration and set it to use only a portion of the available bandwidth.
You should also check your computers for virus or spyware infection.