I've just "upgraded" to 3.3.1.2 for my 2820vn and my SIP had stopped registering. I thought the update news said that this firmware would increase SIP stability not BRICK IT!!!!

Update:
I have reverted back to 3.3.0 and VoIP SIP is magically back up and running.

Does Draytek have any comment to this effect?

I had loads of issues with the 2820 too many to list here, support helped but i got the impression the testing on these routers was poor, I sent my config to them when returning the router to where i bought it, and chased up further info about a month later to support just for my own interest/curiosity.

They replied saying that there was a new f/w 3.3.1.2 which fixed most of my issues. Sorry to hear it doesn't seem to be the case with your post.

I have got SIP stability issues too. It registered then stops, a reboot is neded to get it to register again. Using 3.3.2.1 (Latest UK) firmware