Releans - Communication platform for Messaging and Chatbots
Twilio - Brings voice and messaging to your web and mobile applications.
Plivo - Plivo simplifies your customer engagement.
BudgetSMS.net - Cheap SMS Gateway with simple HTTP API to send SMS messages worldwide.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.