I've resurrected a few calls for friends. I've also made my own calls.
In one case, I had to rub denatured alcohol on the surfaces until whatever was on them came up on a paper towel. It looked like somebody had sneezed on it, and then rubbed it in with a kleenex.
In another case, one side of the call had gone silent. I gave a few passes over the top with 200 grit sandpaper and then used 400 and 600 grit to clean it up. It sounded great after that.
Usually, the call just needs to be retuned. A slight adjustment of the screw will usually do it. Treerooster's admonitions are correct.