Asking for proof of debt is actually a valid tactic.
When I killed my land line and went cellular, oh, 30 years ago, I paid my last phone bill and cancelled.
5 years later I got a call from collections saying I owed for the final bill.
I apologized to them for Qwest wasting their time, told them what was up and asked for proof of debt for which I knew there was none.
Never heard from them again.