Blocking has to wait until after the first ring to get the callerID (that's when it's sent) to know to block the call. You can't block that first ring. (If your carrier blocks calls, that might stop the call from getting to the phone at all, but if you're blocking a lot of calls, they may not block them all - they may have a limit. And if you're trying to block robocalls, it won't work - the "callerID" is a random number for each call. If the same robodialer calls you 5 times in a row, it would have 5 different "callerIDs".)