Hi...if a PI can gain legal entry to a property and with a owner/co owner permission they can install a simple device that can easly record telephone conversations. If they need to listen in to a person with out a phone they can use a Parabolic Microphone (kinda looks like a pistol with saterlite dish on the end) You stand at a distance and point it at the person you want to listen to. They not cheap and hard to conceal but can be very effective. Oh..and txt messages can now be tapped. The target phone (the one you want to snoop on) can be remotley programed (so the target has no idea of this) so that when it sends or receives a txt, a copy is sent to the PI's phone, without the target even knowing. The same goes for mobile calls. Using the same system the Pi gets a message saying a call is going to or from the target phone and he can simply dial a special number and listen in on the call. Find out more at
http://megaspy.net/txt_intercept.php