The setup in this kind of attack is particularly juicy for the attacker: Because of the seed planted in advance, when the target discovers he has a problem, he himself makes the phone call to plead for help. The attacker just sits and waits for the phone to ring, a tactic fondly known in the trade as reverse social engineering. An attacker who can make the target call him gains instant credibility: If I place a call to someone I think is on the help desk, I'm not going to start asking him to prove his identity. That's when the attacker has it made.