Twilio - Brings voice and messaging to your web and mobile applications.
46elks - Adding SMS and voice to your application or service should be easy.
Plivo - Plivo simplifies your customer engagement.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
FreeSWITCH - FreeSWITCH is an open-source media application designed to support popular protools such as SIP and WebRTC and provides a platform to develop voice and video applications.