Who Can I Talk To?

A salesman telephoned a household, and a four year old boy answered:

Salesman: May I speak to your mother?
Boy: She's not here.
Salesman: Well, is anyone else there?
Boy: My sister.
Salesman: O.K., fine. May I speak to her?
Boy: I guess so.

At this point there was a very long silence on the phone. Then:

Boy: Hello?
Salesman: It's you. I thought you were going to get your sister.
Boy: I tried, but I can't get her out of the playpen!