I've tried to do on a 2600v before now and failed, I couldn't find any way for the router to find the gateway of the other router, shame as I have a 2800G but have to use the 2600v as the main router but at a lower speed on the ADSL, I lose about a quarter of my speed due to the 2600 being limited to 8meg.

I've been trying to do exactly this with a 2800V and getting nowhere. My internet connection is via a Cisco ASA so the Draytek itself is not connected to the ADSL. I've put a static route into the Draytek to send traffic for my SIP provider out to the Cisco and I can ping the SIP providers server with no issue. I've put a packet capture on the Cisco firewall and can can see the ping packets but nothing else which suggests to me that the 2800V is completely ignoring the "Route by LAN" option within the SIP setup. It's got me beat.

I gave up and got a 2820Vn. However it is not re-registereing if the SIP drops out. COme on PLEASE Draytek sort this issue.