I used to use ATT Callvantage before it was cancelled as an offering. At that time I switched over to VoipO. I've been using VoipO for over 1yr with satisfactory results. The annual price is very competitive. Their customer service has been helpful. I had only 1 issue that they resolved on their end within a few hours.
They don't have the TOS caps around usage that I've seen on other offerings, well, at least they didn't when I signed up. Customer service is (intended to be) online based but is (has been) very responsive when needed.