Wrong number

On the way to the pool I mentioned to the girls that I didn't have my phone. Q5 started asking questions:

Q5: "What, that means you can't call Daddy?"
E41: "Nope, can't call Daddy"
Q5: "You can't call Mumma?"
E41: "Nope, can't call Mumma."
C6: "Q5, that IS mumma, of course she can't call herself!"
Q5: "I know, it was joke to see if she could call herself! Get it?
E41: "If I called myself I'd just get a busy signal"

Blank stares.

C6: "What is a busy signal?"