Twilio - Brings voice and messaging to your web and mobile applications.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
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.
BulkSMS - SMS messaging service provider offering two-way SMS communication services.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).