A pay telephone housing comprises a large capacity vault having a floor, first and second side walls, and a back wall. The vault includes a front opening and a peripheral rim extending at least partly around the front opening and having a rear edge. A removable vault door is provided for covering and uncovering the front opening and includes first and second locking plates which are operable to be extended to a locking position at least partly behind the rear edge of the peripheral rim to prevent the removable vault door from being removed from the vault. A key operated lock assembly is operable for selectively locking the first and second locking plates in the locking position.