Crunchy Frog, you’re obviously a Monty Python fan. Do you know the Argument sketch?
“Hello, is this customer service?”
“I told you once.”
“… No you haven’t.”
“Yes I have.”
“When?”
“Just now.”
“No you didn’t.”
“Yes I did!”
Repeat ad nauseum. Better yet, when someone calls, berate and insult them viciously. When they get mad and defensive, apologize and say your department is Abuse. Tell them they dialed the wrong number and give them the number they just called. When they call again, proceed with the Argument dialogue above.