Every competent criminal defense attorney will advise their client (or anyone else) NEVER to talk to the police. Law professor James Duane gave a 45-minute lecture on this exact subject. So, obviously if a person with a cop at their door calls up Duane and asks him, "There is a cop at the door and he wants to ask me some questions." Professor Duane would obviously tell the person, "Under no circumstances should you say anything to the police." It would seem unlikely that Duane would be prosecuted for witness tampering.
However, let's imagine that the same person does not call Professor Duane, but calls their brother. Can the brother be charged with witness tampering?
At what point does advising a person not to talk to the police become "witness tampering"?