I have a VTO2202 and have figured out how to ring a traditional doorbell with it. It does take some modification to do it, solder in two leads to sense when the button is pressed, and a voltage sensing relay, but it does work. I haven't played with a SIP server yet, I'm having trouble finding a round tuit.
I also have a DB1C and find it to be an outstanding POS. It drops offline about 200 times, or more, a day. It's on an 18 volt, 30VA transformer so power is not an issue. When I say it "drops offline" what it really is doing is rebooting itself. That resets its configuration so it won't even ring the doorbell unless I go in, 200+ times a day, and reconfigure it. Since I have other things to do in my life than reconfigure a DB1C all day, it only serves as a camera. I definitely can't recommend it to anyone.