I'm afraid to say that I think this effect is pretty permanent
Most of my ivory 706/746s are similarly affected even though they were pristine when put away.
I think the cuplrit is indeed sunlight, but the effects only appear after the phone has been in the dark for a while. I'm guessing this because the example that lived in the dark hallway isn't too bad, but the one that lived on my desk in a south-facing bay window is awful.
I've also noticed the same effect with old computer monitors and other office equipment.
It's rather like with old pianos which have ivory keys. If they've lived in the sun but you carry on keeping the lid open, the keys will stay a nice creamy white colour. But if the piano's player goes away and the lid is left closed for a few years, the keys will be a nasty yellow colour when its opened up again
As for cleaning, use aerosol foam cleaner followed by aerosol household furniture polish, but don't expect them to change colour (unless they're filthy).
Nick.