Twilio - Brings voice and messaging to your web and mobile applications.
Plivo - Plivo simplifies your customer engagement.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
SendBird - SendBird is messaging-as-a-service.
QuickBlox - Cloud communication and data backend.