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.
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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.