Everyone makes mistakes, and usually it haunts us whenever we think we could make a difference, but no one would have planned that out, when you were not available to pick up the call.
This is something we must learn to be at peace with ourselves, at the same time, remember not to make the same mistake.

It's not a condemnation lesson, rather an awareness: that we should never let distraction, depression, or anything negative (that we are never made to receive or carry—but something else more sinister to throw us off from becoming the best version we were designed and created by God) to ever come between our sanity and our focus.

I am sure your friend wouldn't have blamed you. And if you didn't know what the autopsy was about, then there is no way we could have anticipated anything, and over thinking is just going to get things worse.

I hope you are still in good contact with his family (since his father actually called you—meaning you were not blamed), and being appreciative to his family would be the best way to remember the best part of his life and how significant he was for you and his family.