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.
GSC - With GSC, you can pick from any of 14 worldwide datacenters to host your own free voice server.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates