I think the initiation is different for each shaman, but most people do go through a transformative experience before being called.
My personal experiences that lead me to the shamanic path involved mental health, general life dis-ease, a situation that could have ended in near death that woke me up to my life situation, and a rebuilding of my life. I didn't know this was where I was being lead, but can look back and see those turning points in hindsight.
I think too that it's not necessarily a life altering situation that calls you, but something that is with you since you can remember, the life altering situation is a wake up call that it is time to start.
