Although not exactly the same problem, I am seeing registration fails on 2 Yealink phones connected to a VoIP provider through a 2850n.
In the last 24 hours I have done the following: 1) Set SIP registrations to ~100 seconds and 2) turned off UDP Keep alive packets. So far, so good!
I also reboot the router at 6am each morning, which may not be so necessary if the UDP fix is doing the trick.
You may want to switch off the UDP Keep Alive setting as 2 minute SIP registrations should keep the NAT open and stop the Draytek killing off the phone registrations. It may also fix your voice quality problems.

Have you applied QoS?