We use Onebox as well. Simple to manage, forwards to our cell phones when we're not home. But it's not like we're getting lots of calls yet, so I can't speak to how it scales.
It's true, and I missed the part in the post about call volume. Onebox doesn't do much for you if your real problem is getting phones on 20 people's desks to make outbound calls with. But it did totally solve our inbound call problem.