I read this book in the 70's when I was 11 years old and now I'm trying to find it again.
As I remember it, the plot is somewhat like this:
Earth is really polluted and everyone will die. But this organization is secretly selecting people to go into hibernation for a few thousand years until Earth regenerates. Then they will wake up and repopulate the Earth. The selected people get phone calls with riddles and clues that they have to solve (but they don't remember them) until they solve the riddles and find the place where they must go to be a part of this.
This is the book that first made me aware of environmental issues and I would just love to read it again.