Wikipedia
JsSIP
JsSIP is a simple to use library for the programming language JavaScript which takes advantage of latest developments in SIP and WebRTC to provide a fully featured SIP endpoint in any website. With JsSIP any website can get real-time communication features using audio, video and more with just a few lines of code. Using JsSIP it is possible to build SIP user agents that can send and receive audio and video calls, and text messages.