Devils Advocate is quite right in his summary of the situation. Ordering an evacuation is about the biggest decision you will ever have to make in the left seat of an airliner, I can think of no circumstances in which the cabin crew should start an evacuation without first attempting to contact the Captain. If they try and fail to make contact due to a crash impact etc. then quite clearly they are on their own and must get on with what they think is best. The Swiss incident and one a few years ago in my own company clearly showed the fact that the cabin crew are not in a position to make that sort of decision in the vast majority of cases.