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Telephone tapping or wire tapping/wiretapping is when somebody listens to another person's telephone calls. They usually use a listening device called a bug to listen to and record the conversation so that another person can then listen to it later.
This is illegal in many countries, including the UK and US because it means that telephone calls are not private. Sometimes, the police need to 'tap' into telephone calls to catch terrorists or other criminals. Evidence from illegal tops may be used in criminal trials in some countries.