Eavesdropping is intercepting, listening or recording conversation through mechanical means.  It’s illegal in all states.  There are essentially two exceptions, unless you’re in law enforcement.  Either you’re in a one party or all party state.  That means that either one party to the conversation or all parties to the conversation must consent to the eavesdropping.  You can check out your state audio law here: https://www.kirschenbaumesq.com/page/alarm-law-issues.
    Whether you need one or all parties to consent makes a big difference.  In a one party state telephone calls can be recorded or listened to by third parties as long as one party to the conversation consents.  Employers can obtain consent from all employees.  In an all parties state getting the employee’s consent isn’t enough.  Consent needs to be unequivocal.  Signs don’t help and do not constitute consent.  Letting someone know that the call is being monitored probably isn’t enough, though it’s such common practice I wouldn’t be surprised if the issue has been litigated.  
    I am frequently asked if audio can be utilized in various situations.  A retail store; church; school classroom; work office; in common areas in a residential building and in a home.  Usually the answer is no, and the reason is that even if one party to the conversation has consented, in public areas there can always be two people talking who haven’t consented.  That conversation cannot be eavesdropped.  But in someone’s home the issue is a bit more complicated.  Obviously the home owner has consented, but I think the same rules will apply because people do not give up their individual rights just because they are in someone’s home.  More importantly, the same rules apply because none of the statutes that prohibit eavesdropping provide for an exemption for a home.  [if you know of one let me know].  
    How about children? Or someone under disability that requires supervision and care?   Does a parent have the right to eavesdrop when it’s his or her child?  What about installing a covert listening device in your elderly parent’s home so you can listen to the aids caring for your parent?  When confronted with these questions I’ve opined that statutes do not seem to make exceptions for these scenarios, but that common sense and how the recorded device is used may make all the difference.
    New York’s highest court was just presented with the question of whether a father could record a cell phone conversation between his estranged wife [and mother] and her boyfriend where the boyfriend was threatening to beat the father’s five year old son.  The news article doesn’t mention how the recording was accomplished [I haven’t seen the case yet]. Months later a neighbor heard the boyfriend and mother arguing and called police, who arrested the boyfriend and mother.  Only then did the father produce the recording for the police.  The recording was used at the criminal trial.
    There is clearly no statutory authority for this recording and the father clearly violated the eavesdropping law.  However New York’s highest court, by judicial fiat, declared the tape permissible because the father had a “good faith, objectively reasonable basis to believe that it was necessary for the welfare of his son to record the violent conversation he found himself listening to”.   This judicial exception is called the “vicarious consent” doctrine and it was adopted in Utah in 1993 and Ohio in 1998 according to the news article.  
    So we see in this case acceptance of the eavesdropping, otherwise illegal and violation of others' privacy, because it served the welfare of a child.  How the data is used makes all the difference and can overshadow how the eavesdropping was accomplished in the first place.
    Re Zeke Lay's comment that "That has succeeded somewhat as the crowds at the licensing office are awesome to behold as the summer starts.  All these young men with nothing better to do with the life."
    I hope he’s not talking about the door-knockers?  It’s a respectable job, they make tons of money and they learn valuable lessons about business and life.
Julie Jacobson
    I agree that it's a respectable job, provided they adhere to acceptable ethical, legal and moral standards and practices.  Some [maybe not all] engage in deceptive practices, which apparently they are well trained and adept at.  No doubt they make "tons of money".  I don't think anyone in the alarm industry can deny the great success these highly skilled sales people bring to the table.  But the general prevailing perception is that rather than skip the house with the competitor's yard sign or window decal, the door knocker is attracted to those homes and sees them as a ripe target for takeover.