Twilio - Brings voice and messaging to your web and mobile applications.
Ring4 Meeting - Dead simple video conferencing - free, in browser
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Fellow.app - A manager’s co-pilot for 1:1s, meetings, feedback & goals 🚀
Plivo - Plivo simplifies your customer engagement.
openmeet - Account free video conferencing with a rooms component that lets you use VC within a physical meeting room. The server can also be hosted in your own cloud for extra privacy/security.