Building a covert listening device is easier than you might think. All you need is a few household items you probably have lying around your home, a discreet hiding location and, of course, a conversation to spy on.
Sync your Bluetooth headset to the cell phone you no longer use.
Power on the cell phone and wait for it to boot up. In the settings menu, you will see an option that is named something along the lines of "Enhancement option." This is used for connecting Bluetooth devices and headsets. Enable the "Enhancement option," which will allow you to use the Bluetooth headset with the cell phone.
Enable "Automatic answer" in the settings menu. This will allow incoming calls to be answered automatically. Set the ringing volume to 0 and the vibrating alert to "off" if it isn't already set.
Hide your cell phone where the conversation you want to listen in on will be taking place. If it is in a vehicle, hide the cell phone inside the glove compartment. If it will be taking place in a room, hide the cell phone inside of a potted plant or anything that will keep the cell phone hidden.
Call the cell phone when you want to listen in on the conversation and listen through the Bluetooth headset.
The better you hide the cell phone, the more covert it will be. Be creative. For example, if the location of the conversation will be inside of a living room that still has a lot of VHS tapes, find an old VHS tape and place the cell phone inside the tape. Also, be sure the cell phone is normal size; the bigger the cell phone the harder it will be to hide it. If you will be using a regular wired headset be sure to tape the excess wire around the cell phone to make it covert. If the cell phone you will be using is a smart phone, install call recording software so you can listen to the conversation at a later date.
If your cell phone carrier requires that you have SIM cards in the cell phone (such as AT&T), be sure the cell phone that will be listening in on the conversation has its own SIM card. This will ensure that no one knows the cell phone number but you and virtually eliminates the risk of anyone calling the cell phone and listening in on the conversation accidentally. Perform this at your own risk. If someone discovers you are listening (and possibly recording) to a conversation without their knowledge then they have the right to sue you for a "Violation of Privacy" clause.