…a guy running a pre-recorded premium rate sex line business from Southend-on-Sea, goes to New York for some reason, and decides to check his phone lines are working. He buys a prepaid AT&T calling card and dials the number. All is working fine, the girls are panting and talking dirty, but he notices that the call is cheap, too cheap in fact. So cheap that it was costing him less to call the number than he is being paid by BT to receive the call. (Premium rate numbers work such that you rent a number from BT and then get a cut of the extortionate fee the caller pays.)
So the guy gets his colleague/mate to fly out to New York that night and they set up in a huge office where they install hundreds of phones and hire cheap Mexican labourers to call their own phone numbers with stacks of the calling cards which they buy in bulk.
In 3 months they made hundreds of thousands of dollars before some shady Costa Rican mafia types dropped in to ask for a piece of the action. At which point they wrap it up and fly home.