The slogan on the side of the plumber's van says, complete with Belgian accent, "Who's there? It's the plumber." Only it rhymes in French. If you're not interested in a brief linguistic discourse, stop reading now.
"Qui c'est?" is the Belgicism. The French would say, "Qui est-ce? meaning "Who is it?" The Belgian is more like "Who's that?" And l'plombier, rather than the complete and written (in both Belgium and France) le plombier, is how it's actually pronounced--in both Belgium and France. So the rhyme sounds like ki say? say luh plohmbee-ay. Hilarious, n'est-ce pas?