Try shouting first, I know it doesn't travel as well as a whistle and its harder to do, but it gives you a lot more information. It gives you a range, either with in shouting distance or between shouting distance and whistle distance. If the person responds they can fill you in on their situation, whether or not they can walk, and you can play "Marco Polo" until you find each other. Either with a mobile person moving toward you (you are on the trail or at a known spot), or with you moving towards an immobile person. If shouting fails to garner a responce, then try responding with whistle blasts. I would probably move towards the person marking my path with engineer tape or similar. Then evaluate the situation from there. Render any aid I can then lead the person out, or go for or call for help.