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

Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs

Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.

Ameyo - Ameyo is a powerful and highly flexible IP-based contact center software that lets you have a personalized interaction with every customer across multiple channels, thereby driving customer engagement to a level par excellence.

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

Five9 - Five9 provides cloud-based call center software for sales, marketing and support.