Mm. Great prank calls:
Call McDonald's and ask how much their Whoppers are. (Many times when we did this, they'd go check and come back after realizing that's BK)
Call BK and ask how much their BigMacs are *same thing*
Call McD's and ask how much salt they put on their fries because you love their food but recently you were put on a low salt diet.
Call BK and ask why their King looks so freaky. We always got funny responses. Some defensive, some agreed.
Call BK and say you have a McD's giftcard but you'd rather spend it there, at BK, and ask if that's manageable. xD (and vis versa)
I love the whole McD vs BK thing. It makes for fun sleepovers. Especially the late night ones.