When I was a member of the local rescue quad, I was certified in heavy duty rescue which means I got to go up and down cliffs, building, etc. That first step over the edge never got easier but once I was over the side and knew that the ropes would hold me, it was a piece of cake.
Good analogy, I am a rock climber as well. Climbing is no problem, until I get to the top and rappel back down. Letting go of the rock is the hardest thing in the world to do.
It is odd that picking up the phone, instills the same type of fear in a person as stepping off of a cliff.