Twilio - Brings voice and messaging to your web and mobile applications.
AudioOne - AudioOne is the next-generation solution for advanced audio conferencing.
Plivo - Plivo simplifies your customer engagement.
Calliflower - Calliflower is a conference call solution that allows multiple users to dial into a conference bridge, and to conduct web meetings.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
LoopUp - LoopUp provides simple business conference calls and online meetings.