Twilio - Brings voice and messaging to your web and mobile applications.
Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.
Maytapi - Serverless Whatsapp Api with Chat and Agent support.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Tellephant - Helping you build incredible chatbots and integrate WhatsApp, Google, Facebook, SMS and more to your communication stack in seconds. All at the cheapest rates out there.
Melrose Labs SMSC Simulator - SMPP SMSC simulator to test SMS messaging applications.