Bring in the best friend. Add her (then) three year old daughter.
The following conversation ensues:
The phone rings.
Friend (F): Hello?
At the other end's her hubby.
Hubby (H): Hi.. how's your day going?
Daughter (D): Mommy, who's that on the phone?
D: Oh! I want to talk! I want to talk!
F gives D the phone.
D: (Very tentative) Hello...
Then, in a VERY loud aside to F, which H can obviously hear:
D: But Mommy, which Daddy's phone is it?