Season 12, Episode 2
Homer is furious when their telephone area code is changed without his knowing and as such leads a campaign to divide the city of Springfield. Later, when rock legends, The Who, play a rock concert in town, they discover the venue is in the 'old' Springfield, which further fuels the angry mob.
See here for details on The Simpsons Wiki.
See here for the script.
Notes & Screenshots: Lord Heath