Twilio is the 800lb gorilla in this space in terms of buzz, but it's fantastic to see that there's strong competition offering a wider set of countries and better pricing. We're using Twilio because they meet our requirements and the great sign-up promo, but phone numbers are not very sticky.
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.
It's usually possible to move phone numbers between providers within the same country. In the past, phone number prefixes were used by telcos to route the calls meaning that it was very difficult to change your provider and keep your number. Great for the incumbent, not for customers and competitors.
