"A salesman telephone a household, and a four-year-..." joke

A salesman telephone a household, and a four-year-old answered.
Salesman: May I speak to your mother?
Child: She is not here.
Salesman: Well, is anyone else there?
Child: My sister
Salesman: O.K., fine. May I speak to her?
Child: I guess so.
There was a long silence on the other phone. Then:
Child: Hello?
Salesman: It`s you. I thought you were going to call your sister.
Child: I did. The trouble is: I can`t get her out of the playpen.