What are some alternatives?
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.
SendBird - SendBird is messaging-as-a-service.
Twilio - Brings voice and messaging to your web and mobile applications.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
VoxImplant - Voximplant is a software program that allows businesses to automate communications by setting up automatic communication systems in the cloud.
Mirrorfly - A Complete Chat, Video & Audio Calling Solution