What are some alternatives?
SendBird - SendBird is messaging-as-a-service.
Slack - A messaging app for teams who see through the Earth!
Twilio - Brings voice and messaging to your web and mobile applications.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
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.
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.