Little Johnny and Salesman

A salesman telephoned a household, and Little Johnny answered. Salesman: “May I speak to your mother?”
Little Johnny: “She’s not here right now.”
Salesman: “Well, is anyone else there?”
Little Johnny: “My sister.”
Salesman: “O.K., fine. May I speak to her?”
Little Johnny: “I guess so.”
At this point there was a very long silence on the phone.
Then:
Little Johnny: “Hello?”
Salesman: “It’s you. I thought you were going to call your sister.”
Little Johnny: “I tried. But I can’t get her out of the playpen.”