Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:

Abot for Slack - Trusted by over 1000 Slack teams ✅. Abot is a simple and highly configurable app for anonymous feedback messages and more. If you want to create a poll on Slack or use it as an anonymous suggestion box you just need to type a simple command.

Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.

Estherbot - Transform your resume into a bot.

Kannel - Open Source WAP and SMS Gateway

Beep Boop - Ridiculously simple hosting platform for Slack bots.