You can build an inexpensive homemade listening device using a mobile phone. This device will allow you to eavesdrop on conversations of which you are not a part. You can complete this simple process in just a few minutes. Take care to only use this device at a property you own or of which you are allowed access. Otherwise, using it for illegal purposes can get you into trouble.
Turn on your mobile phone, and go to the main menu. From there, go to "Profile Settings," and set the profile to "Silent Mode." Turn the "Vibrate" setting to "Off."
Go back to the main menu, and scroll until you find the "Call Settings." Select "Automatic Answer."
Set the phone in the area on which you wish to listen in. Place it in a hidden location, but keep the microphone uncovered. If you have a headset, you can also connect it to the phone and extend it out so the phone isn't exposed. Again, do not cover the microphone.
Call your mobile phone from a different phone, and listen in on the conversation. If you wish to record, place a tape recorder next to the receiver of your phone.