Mumble - Mumble is an open source, low-latency, high quality voice chat software primarily intended for use...

Twilio - Brings voice and messaging to your web and mobile applications.

Discord - Step up your game with a modern voice & text chat app. Crystal clear voice, multiple server and channel support, mobile apps, and more.

Plivo - Plivo simplifies your customer engagement.

RaidCall - Free & Light. Setting up is fast and easy; with no need to rent or set up servers. Low CPU usage and minimum memory footprint allows for smooth, uninterrupted communication. im. Group Communication.

Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates