How to Eavesdrop on Phone Conversationsby Contributor
Listening in on another person's phone conversation may be easier than you think, but it may also be illegal in certain circumstances. If you're willing to risk angering friends or family members, there are easy ways to monitor their phone conversations.
Pick up the extension. The simplest way to eavesdrop on a phone conversation is to use a handset that is on the same phone line. Quickly press the "Mute" button to block noises on your end of the line from alerting the people who are speaking.
Hang up at the same time as those who are talking. If you hang up earlier, you may alert them to your presence if they hear the click of your handset disconnecting.
Record a conversation when you are a participant only if you live in a one-party consent state. In some states, you may legally record a phone conversation between yourself and another party because, under the one-party consent rule, you are the one party that is consenting. Make sure you know the laws in your state before you try this method (see Resources below).
Purchase a tiny FM (frequency modulation) transmitter and install it in one of your phone handsets to eavesdrop on a phone conversation between two other parties. You can pick up the conversation on a radio in a nearby room. Check with an attorney before doing this and restrict the usage to your home phone. If you need to monitor a child's conversation, this might work.
Attach a pen register to the phone line to record calls only if you have a court order to do so. Law enforcement officers may use a "trap and trace" device to log all incoming and outgoing phone numbers.
Think twice before recording a phone conversation without the permission of the participants. Chances are that you can't use it in court and it may get you in trouble if it is illegal in your state.