A bug or listening device can be detected by recognizing strange noises on the telephone, changes in the volume during conversation and a person not connected with a situation who knows more than they should. These are the most common ways that eavesdroppers are able to listen into business dealings and get confidential information that was not intended for them.
Tapping a phone line is illegal, but it is still sometimes done for the sake of eavesdropping. The phone may make a clicking sound when it is picked up or the person on the other line may sound very far away when they are being spoken to. Law enforcement should be contacted immediately when there is suspicion that a phone line is tapped. While calling the police is the only surefire way to make sure that a phone line is not tapped, it may not always end up with the line being fixed. People who believe that their phone line has been tapped should make sure that they do not say anything on the phone that they would not want others to hear. It is best for people who have a suspected bugged phone line to have a live meeting with the person they want to talk confidentially to.