Apptimate!

Twilio is a web-service API that lets you use your existing web languages and skills to build voice and SMS applications. We focus on building a simple, powerful, pay-as-you-go communications platform so that you can focus on what you do best.

Sinch is a powerful voice and messaging SDK that lets you bring your apps to life with VoIP and WebRTC technology. Simplicity is at its core and you can get your first app up and running in a few lines of code and a matter of minutes.