HOW MANY CALLS IS IT GOING TO TAKE TO CANCELMY CONTRACT

I called on 17/2/2021 to cancel my contract and move to prepaid on the same number - reference ***. I was told I needed to give a month's notice (even though I was out of contract period)and was told my last day of payment would be end March 2021. Ive just checked my app and see I have an invoice due end April 21! I tried calling the call centre twice and they are experiencing high call volumes so I must call back. What poor service! Can they just do their job! FYI I will just keep reversing the debit orders.