This API is available on Firefox OS for internal applications only.
Summary
The onvoicechange property specifies an event listener to receive voicechange events. These events occur whenever the MozMobileConnection.voice connection object changes.
Note: MozMobileConnection inherits from the EventTarget interface, so it's also possible to listen for events by using the addEventListener() method.
Syntax
navigator.mozMobileConnection.onvoicechange = funcRef;
Where funcRef is a function to be called when the voicechange event occurs.
Specification
Not part of any specification.