This entire process occurs before your phone is even aware of the call to decline it. They are taking advantage of the fact that it takes at minimum a few dozen milliseconds to route a call to your phone.
If you have one of the carriers others have mentioned, they are calling your voicemail directly, completely bypassing your phone.
A few seconds actually. There's quite a bit of features that need to be checked along the path (said as someone working in the telecom industry providing name information).
If you have one of the carriers others have mentioned, they are calling your voicemail directly, completely bypassing your phone.