the hint to the code to the locker is in the phone world (walk to the left of the locker and youll see some arms and legs in a specific shape, walk up through the legs)
the code is a combo of 3 numbers in that world that make the same sound (or is repeated in the phone booths), you have to figure out what order they go in (theres a double number)
although you dont need to figure out the locker to progress the game, its just extra content you can do, theres a walkthrough you can download on the game page thatll walk you through what to do in this area