While watching The Mentalist I observed that whenever the agents went to the families for interrogation, they always knocked on the door of the house to declare that they were there. In some of episodes they had this kinda hi-tech devices(like they could communicate through it).
where I live, it is common to have simple door-bells at the entrance, no knocking or that talking devices. So, what do the common people have in the US? No door bells? I don't think they knock these days. If so, then is there any reason why agents always knock the doors in The Mentalist?