What happens to blocked numbers depends on your carrier. There could be a pre-recorded such as: the number isnt reachable, or the number cannot accept calls, etc. Another feature might be caller hears one ring and then the call is disconnected.
You might also check the incoming number against the blocked call list. Sometimes spammers use rotating numbers, that are only digits off, like
999-123-4567 <- blocked number 999-223-4467 <- not yet blocked
