This is wildly inaccurate to the point of being dangerous advice. The goal during a bomb threat call is generally not to challenge, mock, or provoke the caller into a reaction. It is to keep the caller talking for as long as possible and gather information that could help assess the threat and assist law enforcement or security. There is no reliable rule that says a "real terrorist" will hang up if laughed at or that a hoax caller will stay on the line. People making threats behave in many different ways and simplistic tests like this are not a dependable way to determine whether a threat is real.