Invoicing a cancelled insurance.

I cancelled my MTN Insurance and received confirmation of cancellation on the 16th of July 2026. I therefore received an invoice in August which I called MTN on the 18th of Aug notifying them about an invoice of a cancelled insurance and they notified me that they will escalate the matter. Today, in September I received another invoice for this cancelled insurance and the amount is accumulating. What lengths should I go to for MTN to stop invoice a cancelled insurance?