Twilio - Brings voice and messaging to your web and mobile applications.
Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
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.
Kannel - Open Source WAP and SMS Gateway
Maytapi - Serverless Whatsapp Api with Chat and Agent support.
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.