For the BT type 700s and any other phone made of ABS material, you can buy a set of plastic polish. I won't mention the brand name but if you go to
www.oldphones.com you'll see it's in three bottles graded for simple polishing, light scratches and discolouration and heavy scratches and discolouration. It works reasonably well and requires some effort and patience. The main downside is that it's only available from the USA (as far as I know) so you'll have to be prepared to wait and pay a bit for postage.
Sparks.