Perhaps I'm "underthinking" this, but some of the suggestions sound apt to increase risk of introducing bumps in the system. I too am annoyed by spam calls. However, because of what I do for a living I must answer all calls even if I don't recognize the number and even if my phone notes that it is "potential spam." Some of those flagged calls were in fact appropriate and important. It takes only a moment to answer, then disconnect if the call is BS. I can't imagine any significant impact on the hiker's battery life if they did that.