I think that the bottom line is there is no quick and easy way to remove discoloration from plastic phones. I always love the ebay listings showing a faded discolored phone and the description states that it "just needs a good cleaning" as if to imply that it will look like new if you invest 10 or minutes to scrub it down.
The only way used to be sanding and it still is likely the only way in a lot of cases. Sanding probably is a 20 hour or more investment in the phone so you aren't going to do that to try to sell if for $50 on ebaY.
The only phones that are worth this sort of investment of time are rare colors and otherwise very collectible phones, not the run of the mill colored phones.
There have been some successes with retrobright and other bleaching processes but they work best with the lighter colors and you still invest quite a few hours in disassembly, bleaching and reassembly.
There is enough info on the forum on these subjects to keep you reading for hours but I don't think anyone can give you a guaranteed best method to try and unlike some of the ebaY listings suggest, it won't be ready to go in 15 minutes.
Just my opinion on the topic.
Terry
