Billed for no service - and cannot get an invoice out of MTN

In June 2019 we discovered that we had been paying a monthly debit order of some R800 since around 2010, which no longer existed. for 8+ years we had no device whatsoever linked to the account so no calls, no date no nothing. and no invoices. It was our own oversight we thought we had cancelled it. We then cancelled the debit order and the account. Our last debit order was paid at the end of June 2019 and we eventually managed to get proof of cancellation on 19th July 2019. HOWEVER this was not the end - we kept receiving demands for payment, we made query after query and asked for the relevant detailed invoice to see what we were paying for. NONE were supp****-By now they were adding interest to the amount. We continued complaining and visited the local branch who told us the amount was for July as the july debit order had bounced. We again informed everyone that no service was being provided and to supply an invoice indicating what the charges were for. To date 23 Feb 2021 the saga continues with a host of debt collectors on our back and threatening SMS's etc. The amount is not substantial, and it would have been very, very much easier simply to pay the so-called outstanding amount HOWEVER THE PRINICPAL IS we are now handing the matter over to the relevant ombudsman. someone has to make a stand. MTN has a shocking record and all but one of our accounts have been migrated to Vodacom