Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
WhatsMany - Send personalised WhatsApp messages in bulk
Kannel - Open Source WAP and SMS Gateway
LoopMessage - LoopMessage is a perfect macOS tool for personalized mass messaging via iMessage/SMS, WhatsApp, Facebook Messenger, Telegram, Viber, and Skype.
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
Porterhouse - Send mass iMessage and SMS texts from your existing number with a single click!