Never. I doubt my mind works quickly enough for me to have been any good at it.
The closest I ever got was having the ear of the guy who does call them i.e. "Buck, switch up to man the next time we have our ass backed up to the 10yrd line"
They should broadcast every game in slomo.
But since you can't have a 7 hour broadcast, the least they could do is make the attempt to show more replays (which they seem to be slowly abandoning) so that folks actually have a chance to see what is really going on in there.