One of these test numbers, known in the lingo as a loop-around, was particularly useful. Phone phreaks used it as a way to find other phone phreaks to chat with, at no cost to them. Phone phreaks also used it as a way to create a callback number to give to, say, a bank. A social engineer would tell somebody at the bank the phone number to call to reach him at his office. When the bank called back to the test number (loop-around), the phone phreak would be able to receive the call, yet he had the protection of having used a phone number that could not be traced back to him.