Asterisk is a open source communications project that lets you create telephony apps for IP PBXs, VoIP Gateways, etc.

Twilio is a cloud communications platform as a service (PaaS).

TeamWox is a business collaboration software.

FreeSWITCH is a free and open source application server for real-time communication, WebRTC, telecommunications, video and Voice over Internet Protocol (VoIP)…