Twilio - Brings voice and messaging to your web and mobile applications.
CloudTalk - Work locally, grow globally
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.