The Yella Hammer is basically a ceramic synthetic slate.
The crayon is basically wax.
I take glass votive candle holders that have candle remnants in them and put them in the freezer for about an hour. With a screwdriver, the hardened wax plug pops out in one piece. (Thank you Martha Stewart.) I'd put the call in the freezer and then try scraping the hardened wax off with a single edge razor blade. Avoid heat as that will melt the wax into the surface. If you manage to remove most of the hardened wax this way, a green scotch pad should remove the rest and resurface your call. Aren't children wonderful? Gotta love 'em.
"The joy of living is his, who has the heart to demand it." Teddy Roosevelt