Whistles, Smoke and Mirrors - Simple, Effective Low Tech Communications.

Be prepared to communicate for survival. The key to survival after living through a disaster is to stop and think, observe your immediate surroundings for all potential dangers, move quickly to a safe place if in danger, keep things simple, take stock of any resources immediatly at hand , conserve energy and resources, improvise with whatever is close at hand to provide yourself with shelter from heat and cold and find a means to communicate to rescuers as soon as possible.

In the forrest, a whistle can be heard up to 1 kilometer away. Your voice will only carry a few hundred metres and won't last nearly as long. In most survival situations, low tech communication is king.