Slack - A messaging app for teams who see through the Earth!
Twilio - Brings voice and messaging to your web and mobile applications.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Mattermost - Mattermost is an open source alternative to Slack.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates