This article is in need of a technical review.
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.
returnNumber property represents the return call number for the voicemail status.
var nbr = instanceOfMozVoicemailStatus.returnNumber
Return a string, or
null if no return number is provided.
Not part of any specification yet.