I'm actually going to try Plivo really soon for my PlayBook/BB10 app. I was going to use Twilio, but I ran into issues with their voice client. It doesn't seem like they want to resolve it. A pity, because it's likely a small issue with their Flash implementation.

With Plivo, it seems that I can get all the benefits of Twilio, at a better price, and it seems to be more open. Moreover, I can connect to it via SIP. And there are OSS libraries available to help me with that part.