There is not an industry standard for emergency phone placement. It is normally handled on a case by case basis because of the variability of environments, terrain, landscape, etc.
That being said, we do have some “best practices” for placing emergency phones. A good rule of thumb is to have them placed within line of sight from each other so that you can see the next emergency phone from the one you are standing at. If this is not feasible because of man-made or natural obstructions, we recommend every 100 yards.