Account in arrears!

Even 1 star is too much for Cell c ,I recently changed my banking details of course I notified cell c,they even confirmed and sent a mandate to this date my account has not been debited .today I called them to find out why ,there was no answer ,my account is now in arrears and when I asked why must I pay so much money ,I was told that last month was debited they can see the payment but not allocated ,so I am requested to send proof of payment ,what nonsense is that ????I'd rather not pay anything especially because it's not my fault