Sometimes it is very hard to be polite

A recent phone conversation:
Me: Hello
Stranger: Can I speak to Roberta
Lucy: AHHH!
M: I think you have the wrong number (being polite while wrestling the phone away from my baby)
S: Do you think I have the wrong number or do you know it?
M: Well, there is no Roberta here. (trying to keep a smile in my tone :-)
S: Okay, just have some backbone when you say it
M: Bye! (still trying, but barely)