They steal VoIP minutes:
Phreakers have been around since way before Internet telephony, but they now have new medium in which to practice their dark arts. Breaking into Internet phone systems can have big payoff in the sale of black market VoIP minutes. Everything is done over the Internet, from hacking into the gateway to reselling the stolen minutes to crooked wholesalers and who then resell the minutes on phone cards via e-mail to an unsuspecting public.
Industry experts put the number of minutes stolen per month at 200 million; that’s $26 million dollars worth of toll fraud. Telecom fraud is enormously profitable to the practitioners. There are estimates of $35 to $40 billion dollars a year gained from illegal activities. These estimates are probably low given that phone fraud is under-reported.
Wikipedia has some history on the practice of phreaking. Despite the scary name, it is safe for viewing at work.