Billed after cancellation even though the line is inactive

I cancelled my landline account at the beginning of December. I was told that I had to wait a month and still had to pay for that entire month. I waited and paid as was my end od the deal. I received a mail telling me that I needed to pay R500 and something yet the line has been inactive since the 3rd of January. I went into the Galleria store and the lines were as long as usual with staff on cruise mode. A staff member asked what was my issue and after I told her she took my landline number down to check the amount owed on her computer. She came back after a few minutes and told me that my account was not cancelled and that I would need to bring in my cancellation form and ID and follow the long line again and still pay for the account! I will not accept this! This is a breach of contract and downright *******! I will NOT pay for your staff competency! If someone there lost my form Telkom needs to fix it. I have thrown my copy away as the landline is already inactive!!! I called the number and it says it does not exist! I expect a reply soon or I shall be taking legal action. This is absolutely unacceptable!