call.retrieveFromHold() retrieves a call from hold, starts transmitting media again.
If a call was put on hold by call.putOnHold() it can be restored to a normal call again by call.retrieveFromHold().
Please notice: if the call was put on hold by other end it cannot be retrieved by us, only by other end.
None.
Nothing.
call.retrieveFromHold() raises no events, but other end will get a wrtc.onHold() event.