How does artillery work?

I have a few questions about artillery.

What triggers the "artillery fire" warning? I.E. how close to the artillery strike do you have to be to receive the warning (within targeting circle, targeted map square).

Second, how long does it take for your artillery to hit the ground? It seems like there are two barrages, one slow, then one fast.

Are any tanks immune to regular artillery or large caliber? Multiple times I have dropped two strikes in a row with a small circle on a spotted enemy and watched as they hit and did nothing. Is it just a low probability for killing via artillery?

If you confirm an artillery strike, then immediately get hit and explode, does your artillery strike order still go through? I ask because normally when you don't die you can hear the commander calling in the coordinates. Can that process be interrupted or is it just for effect?

Lastly, how exactly do you earn artillery strikes? One per kill? Per score amount? Time delay?