All plastic phones discolor in time, depending on the amount of smoke and sunlight they are exposed to during their lifetime. This can mostly be seen in the Mint Green, Pink variations and Wegdwood phones. But the color White is extremely sensitive to this process.
In the picture you see the shades of white that I encountered in 4 different PTT White phones. The one on the left is from my own collection and close to its original White. The other three are scrap shells with severe damage. But they have 1 thing in common, being the very same initial color: PTT White.
The second shell has some light yellowing, it is still okay to display and you won’t notice it too much, if it is not next to a shiny white one. Number 3 is a bit further in the process and the fourth shell is close to the Candle Glow color. Actually I bought this one as being a Candle Glow 🙁
So when you buy a phone, check the inside and the outside if you are able to. Take a screwdriver with you to open it up. In a lot of cases you will be able to see the color code (520…), although not all phones have it printed. After opening you can easily see the grade of yellowing it has gone through. Discoloring or yellowing is usually not happening on the inside. Sometimes around the holes the process can already be seen.
Also check under the stickers that are applied to the shell. The only way of getting rid of the white circle on the 3rd model is to sand the whole shell. So if you have a phone with a sticker, be carefull, because it is a lot of work.
Never scratch off a sticker. Paper stickers can be removed easily if you apply a wet tissue on top of them for about 15 minutes. Plastic stickers… use your nails to get them off. In both cases you can remove the glue residue with some ‘anti-sticker’ or oily substance. But make sure you clean it right after.