Twilio - Brings voice and messaging to your web and mobile applications.
BroadSoft UC-One - UC-One is a suite of cloud communications offering business phone services, messaging, audio and video conferencing, and more.
Plivo - Plivo simplifies your customer engagement.
MeetingMogul - Join any conference call with one-touch
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
GoToMeeting - GoToMeeting is a web conferencing service offering a range of services which are available on Mac, PC, iOS and Android devices.