Settled account handed over for debt collection a year and a half later

I obtained settlement figures to pay up a contract and close the account. I received the figures, paid the account and forwarded both the figures and the proof of payment to Vodacom. I kept on phoning until I received verbal confirmation that the account had been closed and I was given a reference number for the call. Now about a year and I half later I receive a phone call from a debt collection agency advising there is a substantial amount outstanding that I must pay. I tried to email Vodacom complaints department (email addresses obtained from their call centre) but the email addresses do not exist. What do I do with debt collectors on my back and a possible bad credit record??? I went to a branch to with all my documents and the manager suggested that the only way to get any response from Vodacom was to post on Hello Peter!!!! This was the MANAGER'S advice - even she knows that it is useless trying to complain to Vodacom