What is Double-zero?
n. The last two digits of a cell phone call that ends precisely on a certain minute.
Since the phone company fucks you and rounds up your minutes, it is most economical to terminate a call exactly at a "double-zero" mark.
My conversation with John was wrapping up at 5 minutes and 47 seconds. When he tried to open up a new subject, I stopped him and ended the call right at "double-zero".