This feature is not on a current W3C standards track, but it is supported on the Firefox OS platform. Although implementations may change in the future and it is not supported widely across browsers, it is suitable for use in code dedicated to Firefox OS apps.
window.navigator.mozTelephony.oncallschanged = funcRef;
This is a non standard implementation, but it is discussing at W3C as part of the System Application Working Group.
|Web Telephony API||Editor's Draft||Editor Draft (WIP).|
For obvious reasons, support is primarily expected on mobile browsers.
|Feature||Chrome||Firefox (Gecko)||Internet Explorer||Opera||Safari|
|Basic support||Not supported||Not supported||Not supported||Not supported||Not supported|
|Feature||Android||Firefox Mobile (Gecko)||IE Mobile||Opera Mobile||Safari Mobile|
|Basic support||Not supported||12.0 (12.0)||Not supported||Not supported||Not supported|