So I've noticed that when calling blocked or unavailable numbers, the phone does one ring and then states that the number is blocked or unavailable. This, I'm assuming, is because the initial ring is the phone connecting to the PSTN, and it is only after the phone has connected to the PSTN that it recognizes the fact that the number is unavailable or your phone is blocked, etc.
However, I've also noticed that when calling a dead phone (no battery), the phone goes directly to voicemail. My question, then, is how does this happen? Am I getting something completely wrong here, or does the phone have a way of instantly recognizing when another phone is dead?
I did not find the right solution from the internet