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
Kamailio - Kamailio® (successor of former OpenSER and SER) is an open source SIP server released under...
BulkSMS - SMS messaging service provider offering two-way SMS communication services.