Yes, I also suspect that the yellowing is nicotine and other airborne organics settling on the surface.
While those might bleach with Clorox or peroxide too, I would first scrub the set thoroughly with a good kitchen cleaner. I particularly love Fantastic. It works well on all sorts of organic deposits and has a bleaching component too. I don't know how many gallons of Fantastic I have gone through just cleaning telephone housings and parts. At Costco they have the big refill bottles. I use it on all sorts of parts, even the metal bases, PVC and Neoprene cords, and once a while even on dials before I put them into the ultrasonic cleaner. On nicotine stained housings I brush the surface with a stiff tooth brush under frequent spraying, once a while rinsing in hot water.
While there is probably no reason to believe that black housings do not discolor from flame-retardant fillers in the plastic, the black color easily masks the discoloration, it seems. The main reason for using the oxidizers (bleach, peroxide) is to save the effort of sanding that Harry just described, which is very hard for fine details on the surface, or tight corners and such. But when the discoloration is from airborne deposits, cleaning is easier and quicker than bleaching or sanding.
