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.
emergencyCallsOnly is a read-only property that indicates whether only emergency calls are possible.
Note: This flag is only relevant to voice connections and when
connected is false.
var voiceConnection = navigator.mozMobileConnection.voice.emergencyCallsOnly;
Return a boolean.
Not part of any specification.