Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
StealthChat - StealthChat is Encrypted, Private, Secure Calls and Chats on Android and Apple iOS
Plivo - Plivo simplifies your customer engagement.