What are some alternatives?
Twilio - Brings voice and messaging to your web and mobile applications.
Plivo - Plivo's Voice API enables businesses to build Voice apps to reach customers globally in 200+ countries. Try Plivo's Voice API for free. Instant Sign Up.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.
Sinch - Sinch is a powerful voice and messaging SDK that lets you bring your apps to life with VoIP and...
CALLR - CALLR is an extremely helpful app for those who have to deal with call control protocols for their business. You can create a customized call control system that will perform a wide range of functions.