... not being recognized (when pretending to be MOLLY), because it shows how invisible (metaphorically) she was. The same when MOLLY pretends to be CLEO and no one recognizes her voice. However, at the beginning, we see that GEORGE, CARTER and MR. PERKINS do "see" her. Why can't they recognize her?
shareGEORGE is the only really odd one, because he and Cleo are friends. He should have suspected something was up. Though to be fair, it was a conversation over the phone, and he assumed she would never wear a costume.
CARTER on the other hand probably hasn't talked to her too much, so he just goes with it.
MR. PERKINS has never heard MOLLY before, so he probably assumes the sisters sound alike.