In Chapter 2: Vigrid, City of Deja Vu, after Verse 2, you'll reach the Vigrid Streets area, which is an open courtyard. There are three telephones in this area - standing in front of one of these telephones and inputting a button code results in unlocking an item and deducting its Halo price from your total. The left telephone inset in the stairway is used for unlocking Weapons; the right telephone for Accessories; and the telephone across the courtyard behind a pillar is used for Characters. You will hear a chime if the button code was input successfully.