Besides using your phone or ringing the doorbell, how would you alert them of your presence? And also, this thread isn't to discuss why knocking was made illegal, so please refrain from doing so.
I would probably scream "hello" at the door until they called the cops. Hell, I do that anyway! Oh wait...
I would probably scream "hello" at the door until they called the cops. Hell, I do that anyway! Oh wait...