It's the one where Lisa runs up the phone bill by calling the Corey hotline and the family is upset with her despite the fact that Bart calls Australia, Antarctica, and Argentina while Homer calls Burkina Faso and Disputed Zone and nobody gets upset with them.
Not familiar with the episode, but Simpsons characters have 4 fingers on each hand (counting the thumb of course), and there are only 8 holes on that phone's rotary dial.
Does that mean The Simpsons naturally count in base 8 octal?
Wait, nevermind, there was that episode where Homer learned the real emergency services number is 912.