While fathers are the ones who usually make the jokes, mothers are the ones who, more often than not, misunderstand them. This time it wasn’t the father who made the dad joke, but the kid, and it was actually pretty funny. Too bad the mother didn’t get it.
However, this is a joke that’s better to say face to face, so she didn’t really ruin it. Telling these kind of dad jokes to your mother is always risky, so doing it on the phone? That seems like something that’s bound to fail.