Check whether your router’s firewall has Denial of Service (DoS) attack prevention enabled, and “UDP Flood Attack” in particular enabled.  If so, disable it and see if calls are still cut off.

If this works, you may like to re-enable the UDP Flood Defence check, and adjust the threshold parameter so that calls are not stopped.  Note that each VoIP conversation generates 50 UDP packets/second per if the default 20ms packet size is used.

Your DrayTek router includes defences against several Denial of Service (DoS) attacks, including “UDP flood defence”. If the router receives a large number of UDP packets (over the Threshold value), it will assume that the cause is a hacker attack, and so will block [b]all[/b] UDP packets for a while (the Timeout value).

With a VoIP call, however, the router will be receiving a stream of legitimate RDP packets (using the UDP protocol) containing the digitised voice conversation.  At the default 20ms packet size, 50 packets are sent per second, but the 10ms setting results in 100 packets/second.  If you have 2 simultaneous VoIP conversations the number of VoIP packets is doubled.  And then there can be other applications running on your network which are also receiving UDP packets …

Unfortunately the router doesn’t distinguish between VoIP and other UDP packets; and when the UDP Flood Defence is triggered, the RDP VoIP packets stop for long enough that the VoIP devices think the call was disconnected.

You will therefore need to either disable the UDP Flood Defence, or adjust its Threshold value.