====== onRinging() ======
wrtc.onRinging() is an event function called when other end's phone starts ringing.

You can use this event to indicate to user her call "has gone through", the phone of the person you are calling to has begun ringing. Usually you do it by playing a local "ringing sound" audio file.

===== Occurs =====
After [[ixnx:wrtc:js:api:wrtc:call|wrtc.call()]], when other end's phone starts ringing.

===== Arguments =====
wrtc.onRinging(obj)
  * obj - call object raising the event

===== Return value =====
Ignored.