Why does the Mum answer "City Morgue" in 'Cat in the Hat'?

In the 2003 movie of Dr Seuss' 'Cat in the Hat' the Mum is making rules and says to Conrad (the son):

Mom: Well, if you're both staying, remember the rules; Conrad, no playing ball in the house, no fighting, no answering the phone: "City Morgue".

Why does she pick up the phone and say city morgue? This quote has plagued me for 15 years and I need an answer. I answer my phone like this but I don't even know why. Is this an American joke I'm missing?

Heres the quote in context: https://www.imdb.com/title/tt0312528/quotes/qt0356642