TapTalk.io - TapTalk.io is a complete in-app chat SDK and messaging API - Easy integration and customizeable.
Twilio - Brings voice and messaging to your web and mobile applications.
Lets Chat - Open source self-hosted chat app for small teams
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
WeChat API - WeChat API and SDK allow developers to access and integrate the functionality of WeChat with other applications.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates