Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Asterisk - An open source telephony switching and private branch exchange service for Linux.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
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.
Plivo - Plivo simplifies your customer engagement.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.