If you want to be on the eye-bleeding edge of scalable VoIP, check out Whistle (http://www.2600hz.org/about-whistle/). Host your APIs (REST and AMQP), choose your carriers, and build your whapps on a completely open-source project. We will gladly host and/or manage your servers as well.
Whereas Twilio/Tropo/Plivio give you ways to manage just call handling, Whistle aims to expose phone provisioning, call handling, server and cluster management, and more. We are open sourcing the PBX tools, a trunking platform (see it live at http://store.2600hz.com), among others that are in development.
That said, we're really excited with plivio's release and hope to either work with them to integrate their app as a whapp on our platform, or fork the project and do it ourselves (if we ever have time)!