Asterisk - An open source telephony switching and private branch exchange service for Linux.
Twilio - Brings voice and messaging to your web and mobile applications.
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.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.