Couldn't he have just given it to Costello, then Costello gets all his guys together, French calls the number and they just see whose phone rings? Boom there's your rat.
shareHe didn’t have the number until after Costello died.
shareI think the only reason he doesn't is because, immediately after calling the number, he looks through the SIU file and sees that picture of Costello talking to Lazlo and other stuff indicating that he's an FBI informant.
He therefore thinks twice about helping Costello and it contributes to his decision to "take him down" when he knows he's going to the warehouse