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.
MozVoicemail object you can use to handle the access to the voicemail functionality from the RIL of a Firefox OS phone.
var voicemail = window.navigator.mozVoicemail;
navigator.mozVoicemail is a
Not part of any specification.