Twilio - Brings voice and messaging to your web and mobile applications.
Call2friends - High-quality cheap and free international calls online to any mobile or landline numbers from internet and free calling app - cheaper than other VoiP providers. Try it now!
Plivo - Plivo simplifies your customer engagement.
Make Free Calls Online - Make Free Calls Online is a web application that, as the name suggests, allows its users to make international and local calls for free.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Poptox - Poptox is a web application where you can communicate with people in every corner of the world.