What happens if you reset your router according to instructions?

Like I said, I have OBi with Voipo service and never had this problem. Did you check the settings that I listed? Are they set to default?

Personally, I don't like DD-WRT; I prefer OpenWRT - but now I have a very primitive router that is built into DSL modem, and I still don't have a problem. And we make 2-3 calls every day that are more than 15 minutes. So, I would look closer to OBi settings, and you can always ask questions on OBi forum - there are a lot of friendly knowledgeable folks there.

As far as forwarding ports 5000-65000, I am at a loss how it could relate to getting disconnected. Unless, perhaps, "disconnected" really means that you don't hear the other party. Still, the other party would hear you! In short, I think you are solving the wrong problem. But I may be wrong...